<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <title>Percentage by Mass: H2O</title>
  <link rel="stylesheet" href="{{VENDOR_CDN}}/katex.min.css">
  <script src="{{VENDOR_CDN}}/katex.min.js"></script>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      -webkit-tap-highlight-color: transparent;
    }

    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: transparent;
      color: #1e2945;
      padding: 16px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100%;
    }

    .wrapper {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      gap: 14px;
    }

    /* Header */
    .header-row {
      display: flex;
      justify-content: space-between;
      align-items: baseline;
      font-weight: 700;
      font-size: 15px;
      padding: 0 4px;
    }

    .mol-title {
      font-size: 18px;
      color: #0f172a;
    }

    .mol-mass {
      font-size: 14px;
      color: #64748b;
      font-weight: 600;
    }

    /* Stacked Bar */
    .bar-container {
      position: relative;
      width: 100%;
      height: 48px;
      background: #f1f5f9;
      border-radius: 12px;
      overflow: hidden;
      display: flex;
      box-shadow: inset 0 2px 4px rgba(0, 0, 0, 0.06);
    }

    .segment {
      height: 100%;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      color: #ffffff;
      font-weight: 700;
      font-size: 14px;
      transition: opacity 0.2s ease, filter 0.2s ease;
      position: relative;
      user-select: none;
    }

    .segment:hover {
      filter: brightness(1.05);
    }

    .segment.seg-h {
      width: 11.11%;
      background: #0284c7;
    }

    .segment.seg-o {
      width: 88.89%;
      background: #ea580c;
    }

    .segment.inactive {
      opacity: 0.45;
      filter: grayscale(0.2);
    }

    .segment-label {
      white-space: nowrap;
      pointer-events: none;
    }

    /* Element Selector Pills */
    .pills {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 8px;
    }

    .pill-btn {
      border: none;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 10px;
      padding: 8px 10px;
      font-size: 13px;
      font-weight: 600;
      color: #475569;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: space-between;
      transition: all 0.2s ease;
    }

    .pill-btn .dot {
      width: 8px;
      height: 8px;
      border-radius: 50%;
      display: inline-block;
      margin-right: 6px;
    }

    .pill-btn.active-h {
      background: #f0f9ff;
      border-color: #0284c7;
      color: #0369a1;
      box-shadow: 0 0 0 1px #0284c7;
    }

    .pill-btn.active-o {
      background: #fff7ed;
      border-color: #ea580c;
      color: #c2410c;
      box-shadow: 0 0 0 1px #ea580c;
    }

    /* Calculation Callout Card */
    .calc-card {
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 14px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      gap: 8px;
      min-height: 86px;
    }

    .calc-title {
      font-size: 12px;
      text-transform: uppercase;
      letter-spacing: 0.05em;
      font-weight: 700;
      color: #64748b;
    }

    .math-display {
      font-size: 16px;
      color: #0f172a;
    }

    @media (max-width: 380px) {
      .bar-container {
        height: 42px;
      }
      .math-display {
        font-size: 14px;
      }
    }
  </style>
</head>
<body>
  <div class="wrapper">
    
    <div class="header-row">
      <div class="mol-title"><span id="formula-title"></span></div>
      <div class="mol-mass"><span id="mass-title"></span></div>
    </div>

    
    <div class="bar-container" id="barContainer">
      <div class="segment seg-h" id="segH" title="Hydrogen: 11.1%">
        <span class="segment-label">H</span>
      </div>
      <div class="segment seg-o" id="segO" title="Oxygen: 88.9%">
        <span class="segment-label">Oxygen (88.9%)</span>
      </div>
    </div>

    
    <div class="pills">
      <button class="pill-btn" id="btnH">
        <span><span class="dot" style="background:#0284c7;"></span>Hydrogen</span>
        <span>11.1%</span>
      </button>
      <button class="pill-btn active-o" id="btnO">
        <span><span class="dot" style="background:#ea580c;"></span>Oxygen</span>
        <span>88.9%</span>
      </button>
    </div>

    
    <div class="calc-card">
      <div class="calc-title" id="calcTitle">Percentage by Mass</div>
      <div class="math-display" id="mathTarget"></div>
    </div>
  </div>

  <script>
    const data = {
      O: {
        title: "Oxygen in H₂O",
        math: "\\%\\text{ O} = \\left( \\frac{1 \\times 16.0}{18.0} \\right) \\times 100 = 88.9\\%",
        class: "active-o"
      },
      H: {
        title: "Hydrogen in H₂O",
        math: "\\%\\text{ H} = \\left( \\frac{2 \\times 1.0}{18.0} \\right) \\times 100 = 11.1\\%",
        class: "active-h"
      }
    };

    let selected = "O";

    function renderKaTeX() {
      if (window.katex) {
        katex.render("\\text{H}_2\\text{O}", document.getElementById('formula-title'), { throwOnError: false });
        katex.render("M_r = 18.0", document.getElementById('mass-title'), { throwOnError: false });
        katex.render(data[selected].math, document.getElementById('mathTarget'), { displayMode: true, throwOnError: false });
      } else {
        document.getElementById('formula-title').textContent = "H₂O";
        document.getElementById('mass-title').textContent = "Mr = 18.0";
        document.getElementById('mathTarget').textContent = selected === 'O' 
          ? "(1 × 16.0 / 18.0) × 100 = 88.9%" 
          : "(2 × 1.0 / 18.0) × 100 = 11.1%";
      }
    }

    function updateState(elKey) {
      selected = elKey;
      const segH = document.getElementById('segH');
      const segO = document.getElementById('segO');
      const btnH = document.getElementById('btnH');
      const btnO = document.getElementById('btnO');
      const calcTitle = document.getElementById('calcTitle');

      if (selected === 'O') {
        segO.classList.remove('inactive');
        segH.classList.add('inactive');
        btnO.className = 'pill-btn active-o';
        btnH.className = 'pill-btn';
        calcTitle.textContent = "Oxygen Calculation";
      } else {
        segH.classList.remove('inactive');
        segO.classList.add('inactive');
        btnH.className = 'pill-btn active-h';
        btnO.className = 'pill-btn';
        calcTitle.textContent = "Hydrogen Calculation";
      }

      renderKaTeX();
    }

    document.getElementById('segH').addEventListener('click', () => updateState('H'));
    document.getElementById('segO').addEventListener('click', () => updateState('O'));
    document.getElementById('btnH').addEventListener('click', () => updateState('H'));
    document.getElementById('btnO').addEventListener('click', () => updateState('O'));

    window.addEventListener('DOMContentLoaded', () => {
      renderKaTeX();
      updateState('O');
    });
  </script>
</body>
</html>

Calculating Percentage by Mass of an Element in a Compound

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <link rel="stylesheet" href="{{VENDOR_CDN}}/katex.min.css">
  <script src="{{VENDOR_CDN}}/katex.min.js"></script>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
      color: #1e2945;
      background: #ffffff;
      padding: 12px;
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    /* Top Nav / Molecule Bar */
    .top-bar {
      display: flex;
      justify-content: space-between;
      align-items: center;
      flex-wrap: wrap;
      gap: 8px;
    }
    .molecule-pill {
      font-size: 16px;
      font-weight: 700;
      background: #f1f5f9;
      padding: 6px 12px;
      border-radius: 20px;
      border: 1px solid #e2e8f0;
      color: #1e2945;
    }
    .step-buttons {
      display: flex;
      gap: 6px;
    }
    .step-btn {
      background: #f8fafc;
      border: 1px solid #cbd5e1;
      color: #64748b;
      font-size: 13px;
      font-weight: 600;
      padding: 5px 10px;
      border-radius: 6px;
      cursor: pointer;
      transition: all 0.2s ease;
    }
    .step-btn.active {
      background: #2563eb;
      border-color: #2563eb;
      color: #ffffff;
      box-shadow: 0 2px 6px rgba(37,99,235,0.25);
    }

    /* Steps Grid */
    .steps-grid {
      display: grid;
      grid-template-columns: repeat(3, 1fr);
      gap: 10px;
    }
    @media (max-width: 520px) {
      .steps-grid {
        grid-template-columns: 1fr;
        gap: 8px;
      }
    }

    .step-card {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 10px;
      padding: 10px 12px;
      display: flex;
      flex-direction: column;
      gap: 6px;
      transition: all 0.3s ease;
      position: relative;
    }
    .step-card.active {
      background: #ffffff;
      border-color: #2563eb;
      box-shadow: 0 0 0 2px rgba(37,99,235,0.15), 0 4px 12px rgba(0,0,0,0.04);
    }
    .step-card.success.active {
      border-color: #16a34a;
      box-shadow: 0 0 0 2px rgba(22,163,74,0.15), 0 4px 12px rgba(0,0,0,0.04);
    }

    .step-header {
      display: flex;
      align-items: center;
      justify-content: space-between;
    }
    .step-num {
      font-size: 11px;
      font-weight: 700;
      letter-spacing: 0.5px;
      text-transform: uppercase;
      color: #64748b;
    }
    .step-card.active .step-num {
      color: #2563eb;
    }
    .step-card.success.active .step-num {
      color: #16a34a;
    }

    .step-title {
      font-size: 14px;
      font-weight: 700;
      color: #1e2945;
    }
    .math-row {
      font-size: 14px;
      line-height: 1.4;
      min-height: 48px;
      display: flex;
      align-items: center;
      color: #1e2945;
    }

    /* Proportional Bar */
    .vis-container {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 10px;
      padding: 10px 12px;
      display: flex;
      flex-direction: column;
      gap: 8px;
    }
    .bar-wrap {
      display: flex;
      height: 28px;
      border-radius: 6px;
      overflow: hidden;
      font-size: 12px;
      font-weight: 700;
      color: #ffffff;
    }
    .bar-fe {
      width: 70%;
      background: #2563eb;
      display: flex;
      align-items: center;
      justify-content: center;
      transition: background 0.3s ease;
    }
    .bar-o {
      width: 30%;
      background: #94a3b8;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    /* Summary Badge */
    .summary-badge {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
      background: #ecfdf5;
      border: 1px solid #bbf7d0;
      border-radius: 8px;
      padding: 8px 12px;
      font-size: 15px;
      font-weight: 700;
      color: #16a34a;
      text-align: center;
    }
  </style>
</head>
<body>

  <div class="top-bar">
    <div class="molecule-pill" id="mol-label">Fe₂O₃</div>
    <div class="step-buttons">
      <button class="step-btn active" data-step="0">1. Total Mᵣ</button>
      <button class="step-btn" data-step="1">2. Iron Mass</button>
      <button class="step-btn" data-step="2">3. % Mass</button>
    </div>
  </div>

  <div class="steps-grid">
    
    <div class="step-card active" id="card-0">
      <div class="step-header">
        <span class="step-num">Step 1</span>
      </div>
      <div class="step-title">Total Mᵣ</div>
      <div class="math-row" id="math-1"></div>
    </div>

    
    <div class="step-card" id="card-1">
      <div class="step-header">
        <span class="step-num">Step 2</span>
      </div>
      <div class="step-title">Iron Mass</div>
      <div class="math-row" id="math-2"></div>
    </div>

    
    <div class="step-card success" id="card-2">
      <div class="step-header">
        <span class="step-num">Step 3</span>
      </div>
      <div class="step-title">Percentage</div>
      <div class="math-row" id="math-3"></div>
    </div>
  </div>

  
  <div class="vis-container">
    <div class="bar-wrap">
      <div class="bar-fe" id="bar-fe">Iron: 112 (70%)</div>
      <div class="bar-o">Oxygen: 48 (30%)</div>
    </div>
  </div>

  
  <div class="summary-badge">
    <span>✦ 70% Iron by mass</span>
  </div>

  <script>
    // Render KaTeX expressions safely
    function renderMath() {
      if (window.katex) {
        katex.render("\\text{Fe}_2\\text{O}_3", document.getElementById('mol-label'));
        katex.render("(2 \\times 56) + (3 \\times 16) = \\mathbf{160}", document.getElementById('math-1'));
        katex.render("2 \\times 56 = \\mathbf{112}", document.getElementById('math-2'));
        katex.render("\\frac{112}{160} \\times 100 = \\mathbf{70\\%}", document.getElementById('math-3'));
      } else {
        document.getElementById('math-1').textContent = "(2 × 56) + (3 × 16) = 160";
        document.getElementById('math-2').textContent = "2 × 56 = 112";
        document.getElementById('math-3').textContent = "(112 / 160) × 100 = 70%";
      }
    }

    renderMath();

    // Step Interaction Logic
    let currentStep = 0;
    const cards = [
      document.getElementById('card-0'),
      document.getElementById('card-1'),
      document.getElementById('card-2')
    ];
    const buttons = document.querySelectorAll('.step-btn');
    let autoPlayTimer = null;

    function setStep(idx) {
      currentStep = idx;
      cards.forEach((card, i) => {
        card.classList.toggle('active', i === idx);
      });
      buttons.forEach((btn, i) => {
        btn.classList.toggle('active', i === idx);
      });
    }

    buttons.forEach((btn, idx) => {
      btn.addEventListener('click', () => {
        clearInterval(autoPlayTimer);
        setStep(idx);
      });
    });

    // Auto-advance calmly every 4 seconds
    autoPlayTimer = setInterval(() => {
      setStep((currentStep + 1) % 3);
    }, 4000);
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }

    body {
      width: 100%;
      min-height: 100%;
      background: #f8f9fa;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      padding: 16px 12px;
    }

    .calc-container {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 12px;
    }

    /* Top Formula Header */
    .formula-badge {
      background: #ffffff;
      border: 1px solid #e6e6e6;
      border-radius: 20px;
      padding: 6px 18px;
      font-size: 15px;
      font-weight: 700;
      letter-spacing: 0.5px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.04);
      display: inline-flex;
      align-items: baseline;
      gap: 4px;
    }

    .formula-badge sub {
      font-size: 11px;
      line-height: 0;
      position: relative;
      bottom: -2px;
    }

    /* Split Branches Container */
    .branches-grid {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 12px;
      width: 100%;
    }

    .branch-card {
      background: #ffffff;
      border-radius: 12px;
      padding: 12px 10px;
      display: flex;
      flex-direction: column;
      align-items: center;
      text-align: center;
      border: 1px solid #e6e6e6;
      box-shadow: 0 1px 4px rgba(0,0,0,0.03);
    }

    .branch-card.hydrogen {
      border-top: 4px solid #38bdf8;
      background: linear-gradient(180deg, #f0f9ff 0%, #ffffff 40%);
    }

    .branch-card.oxygen {
      border-top: 4px solid #fb7185;
      background: linear-gradient(180deg, #fff1f2 0%, #ffffff 40%);
    }

    .element-pill {
      font-size: 13px;
      font-weight: 700;
      padding: 2px 10px;
      border-radius: 10px;
      margin-bottom: 8px;
    }

    .hydrogen .element-pill {
      background: #e0f2fe;
      color: #0369a1;
    }

    .oxygen .element-pill {
      background: #ffe4e6;
      color: #be123c;
    }

    .calc-step {
      font-size: 13px;
      color: #64748b;
      margin-bottom: 4px;
      font-weight: 500;
    }

    .subtotal {
      font-size: 18px;
      font-weight: 800;
    }

    .hydrogen .subtotal {
      color: #0284c7;
    }

    .oxygen .subtotal {
      color: #e11d48;
    }

    /* Connector area */
    .operator {
      display: flex;
      align-items: center;
      justify-content: center;
      width: 28px;
      height: 28px;
      border-radius: 50%;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      font-size: 16px;
      font-weight: 700;
      color: #64748b;
      box-shadow: 0 1px 2px rgba(0,0,0,0.04);
      margin: -4px 0;
      z-index: 2;
    }

    /* Total Result Box */
    .total-box {
      width: 100%;
      background: #ffffff;
      border: 1.5px solid #cbd5e1;
      border-radius: 12px;
      padding: 12px 16px;
      display: flex;
      justify-content: space-between;
      align-items: center;
      box-shadow: 0 2px 6px rgba(0,0,0,0.04);
    }

    .total-label {
      font-size: 14px;
      font-weight: 600;
      color: #475569;
      display: flex;
      align-items: baseline;
      gap: 3px;
    }

    .total-label sub {
      font-size: 11px;
      line-height: 0;
      position: relative;
      bottom: -2px;
    }

    .total-val {
      font-size: 22px;
      font-weight: 800;
      color: #1e2945;
      display: flex;
      align-items: baseline;
      gap: 6px;
    }

    .sum-detail {
      font-size: 13px;
      font-weight: 500;
      color: #94a3b8;
    }

    @media (max-width: 360px) {
      .calc-step {
        font-size: 11px;
      }
      .subtotal {
        font-size: 16px;
      }
      .total-val {
        font-size: 19px;
      }
    }
  </style>
</head>
<body>

  <div class="calc-container">
    
    <div class="formula-badge">
      <span>Formula:</span>
      <span>H<sub>2</sub>O</span>
    </div>

    
    <div class="branches-grid">
      
      <div class="branch-card hydrogen">
        <div class="element-pill">2 × H</div>
        <div class="calc-step">2 × 1.0</div>
        <div class="subtotal">2.0</div>
      </div>

      
      <div class="branch-card oxygen">
        <div class="element-pill">1 × O</div>
        <div class="calc-step">1 × 16.0</div>
        <div class="subtotal">16.0</div>
      </div>
    </div>

    
    <div class="operator">+</div>

    
    <div class="total-box">
      <div class="total-label">
        <span>Total M<sub>r</sub></span>
      </div>
      <div class="total-val">
        <span class="sum-detail">2.0 + 16.0 =</span>
        <span>18.0</span>
      </div>
    </div>
  </div>

</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Stoichiometry Calculation - Mg to MgO</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }

    body {
      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
      background: transparent;
      color: #0f172a;
      padding: 10px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: flex-start;
      min-height: 100vh;
      max-width: 540px;
      margin: 0 auto;
    }

    .diagram-container {
      width: 100%;
      display: flex;
      flex-direction: column;
      gap: 10px;
    }

    /* Equation Header */
    .header {
      width: 100%;
      text-align: center;
    }

    .equation-bar {
      display: inline-flex;
      align-items: center;
      justify-content: center;
      gap: 6px;
      background: #f1f5f9;
      border-radius: 20px;
      padding: 6px 16px;
      font-size: 15px;
      font-weight: 700;
      color: #1e293b;
      letter-spacing: 0.2px;
    }

    .equation-bar .coef {
      color: #2563eb;
      font-weight: 800;
    }

    .equation-bar .arrow {
      color: #64748b;
      margin: 0 4px;
      font-size: 16px;
    }

    /* Flow Nav Tabs */
    .flow-nav {
      display: flex;
      width: 100%;
      gap: 6px;
    }

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      min-height: 250px;
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      display: inline-flex;
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      font-size: 11px;
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      gap: 2px;
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      font-size: 11px;
      color: #64748b;
      font-weight: 500;
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      gap: 6px;
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      text-align: center;
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      font-size: 13px;
      line-height: 1.1;
    }
    .frac > .num {
      border-bottom: 1.5px solid #475569;
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    .frac > .den {
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    }

    /* Result Banner */
    .result-banner {
      display: flex;
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    .dot {
      width: 8px;
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        padding: 3px 6px;
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        font-size: 13.5px;
      }
      .result-banner {
        font-size: 11px;
      }
    }
  </style>
</head>
<body>

  <div class="diagram-container">
    
    <div class="header">
      <div class="equation-bar">
        <span><span class="coef">2</span>Mg + O<sub>2</sub></span>
        <span class="arrow">→</span>
        <span><span class="coef">2</span>MgO</span>
      </div>
    </div>

    
    <div class="flow-nav" role="tablist">
      <button class="flow-tab active-1" id="tab-0" onclick="goToStep(0)" role="tab" aria-selected="true">
        <span class="step-num">1</span> Mass → Mol
      </button>
      <button class="flow-tab" id="tab-1" onclick="goToStep(1)" role="tab" aria-selected="false">
        <span class="step-num">2</span> Mole Ratio
      </button>
      <button class="flow-tab" id="tab-2" onclick="goToStep(2)" role="tab" aria-selected="false">
        <span class="step-num">3</span> Mol → Mass
      </button>
    </div>

    
    <div class="card-stage">
      
      
      <div class="step-card active" id="card-0">
        <div class="card-top">
          <span class="badge badge-blue">Step 1</span>
          <span class="tag">Moles of Reactant (Mg)</span>
        </div>

        <div class="visual-bridge">
          <div class="bridge-item">
            <span class="bridge-sub">Given Mass</span>
            <span class="bridge-val">12.15 g Mg</span>
          </div>
          <span class="bridge-op op-blue">÷ 24.3 g/mol</span>
          <div class="bridge-item">
            <span class="bridge-sub">Moles</span>
            <span class="bridge-val">0.50 mol Mg</span>
          </div>
        </div>

        <div class="math-box">
          <div class="math-formula">
            <span><i>n</i>(Mg) = </span>
            <span class="frac">
              <span class="num">12.15 g</span>
              <span class="den">24.3 g/mol</span>
            </span>
            <span> = <strong>0.50 mol</strong></span>
          </div>
        </div>

        <div class="result-banner res-blue">
          <span>Molar mass <i>M</i>(Mg) = 24.3 g/mol</span>
          <strong><i>n</i> = 0.50 mol</strong>
        </div>
      </div>

      
      <div class="step-card" id="card-1">
        <div class="card-top">
          <span class="badge badge-orange">Step 2</span>
          <span class="tag">Stoichiometric Mole Ratio</span>
        </div>

        <div class="visual-bridge">
          <div class="bridge-item">
            <span class="bridge-sub">Reactant</span>
            <span class="bridge-val">2 mol Mg</span>
          </div>
          <span class="bridge-op op-orange">2 : 2 → 1 : 1</span>
          <div class="bridge-item">
            <span class="bridge-sub">Product</span>
            <span class="bridge-val">2 mol MgO</span>
          </div>
        </div>

        <div class="math-box">
          <div class="math-formula">
            <span><i>n</i>(MgO) = 0.50 mol × </span>
            <span class="frac">
              <span class="num">2 mol MgO</span>
              <span class="den">2 mol Mg</span>
            </span>
            <span> = <strong>0.50 mol</strong></span>
          </div>
        </div>

        <div class="result-banner res-orange">
          <span>1 : 1 Mole Ratio (2 Mg : 2 MgO)</span>
          <strong><i>n</i>(MgO) = 0.50 mol</strong>
        </div>
      </div>

      
      <div class="step-card" id="card-2">
        <div class="card-top">
          <span class="badge badge-green">Step 3</span>
          <span class="tag">Mass of Product (MgO)</span>
        </div>

        <div class="visual-bridge">
          <div class="bridge-item">
            <span class="bridge-sub">Moles</span>
            <span class="bridge-val">0.50 mol MgO</span>
          </div>
          <span class="bridge-op op-green">× 40.3 g/mol</span>
          <div class="bridge-item">
            <span class="bridge-sub">Final Mass</span>
            <span class="bridge-val">20.15 g MgO</span>
          </div>
        </div>

        <div class="math-box">
          <div class="math-formula">
            <span><i>m</i>(MgO) = 0.50 mol × 40.3 g/mol = <strong>20.15 g</strong></span>
          </div>
        </div>

        <div class="result-banner res-green">
          <span><i>M</i>(MgO) = 24.3 + 16.0 = 40.3 g/mol</span>
          <strong><i>m</i> = 20.15 g</strong>
        </div>
      </div>

    </div>

    
    <div class="controls">
      <button class="ctrl-btn" onclick="prevStep()">← Prev</button>
      <div class="dots-wrap">
        <div class="dot active" id="dot-0" style="background: #2563eb;" onclick="goToStep(0)" title="Step 1"></div>
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        <div class="dot" id="dot-2" onclick="goToStep(2)" title="Step 3"></div>
      </div>
      <button class="ctrl-btn" onclick="nextStep()">Next →</button>
    </div>
  </div>

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</body>
</html>

Stoichiometric Calculations from Balanced Equations

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <link rel="stylesheet" href="{{VENDOR_CDN}}/katex.min.css">
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  <style>
    * { 
      box-sizing: border-box; 
      margin: 0; 
      padding: 0; 
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; 
    }

    body {
      background: transparent;
      color: #0f172a;
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      justify-content: flex-start;
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      user-select: none;
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    .main-container {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      gap: 10px;
    }

    .nav-bar {
      display: flex;
      gap: 6px;
      width: 100%;
    }

    .step-btn {
      flex: 1;
      padding: 8px 4px;
      font-size: 12px;
      font-weight: 700;
      background: #f1f5f9;
      color: #1e293b;
      border: 1px solid #cbd5e1;
      border-radius: 6px;
      cursor: pointer;
      text-align: center;
      transition: background 0.15s, color 0.15s, border-color 0.15s;
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    .step-btn.active {
      background: #1d4ed8;
      color: #ffffff;
      border-color: #1d4ed8;
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    .play-btn {
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      font-size: 13px;
      font-weight: 700;
      background: #f8fafc;
      color: #0f172a;
      border: 1px solid #94a3b8;
      border-radius: 6px;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      min-width: 40px;
    }

    .stage {
      position: relative;
      width: 100%;
      min-height: 270px;
    }

    .step-panel {
      position: absolute;
      inset: 0;
      display: flex;
      flex-direction: column;
      gap: 10px;
      opacity: 0;
      pointer-events: none;
      visibility: hidden;
      transition: opacity 0.25s ease, visibility 0.25s;
    }

    .step-panel.active {
      position: relative;
      opacity: 1;
      pointer-events: auto;
      visibility: visible;
    }

    .step-header {
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 2px 0;
    }

    .flow-step-tag {
      font-size: 13px;
      font-weight: 800;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      color: #0f172a;
    }

    .step-indicator {
      font-size: 12px;
      font-weight: 700;
      color: #1d4ed8;
      background: #eff6ff;
      padding: 2px 8px;
      border-radius: 12px;
      border: 1px solid #bfdbfe;
    }

    .calc-box {
      background: #f8fafc;
      border: 1px solid #cbd5e1;
      border-radius: 8px;
      padding: 10px 12px;
      font-size: 14px;
      line-height: 1.5;
      color: #0f172a;
      display: flex;
      flex-direction: column;
      gap: 6px;
    }

    .fallback-formula {
      font-size: 13px;
      color: #0f172a;
    }

    .highlight {
      color: #1d4ed8;
      font-weight: 700;
    }

    .accent {
      color: #c2410c;
      font-weight: 700;
    }

    /* Particle visual representations */
    .visual-row {
      display: flex;
      align-items: center;
      justify-content: space-around;
      background: #f8fafc;
      border: 1px solid #cbd5e1;
      border-radius: 8px;
      padding: 14px 10px;
      gap: 6px;
    }

    .cluster {
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 6px;
      text-align: center;
    }

    .fe2o3-unit {
      position: relative;
      width: 72px;
      height: 52px;
    }

    .atom {
      position: absolute;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 10px;
      font-weight: 800;
      color: #ffffff;
      box-shadow: inset -1px -1px 3px rgba(0,0,0,0.35), 0 2px 4px rgba(0,0,0,0.15);
    }

    .atom-fe {
      width: 28px;
      height: 28px;
      background: #c2410c;
      z-index: 2;
    }

    .atom-o {
      width: 20px;
      height: 20px;
      background: #dc2626;
      z-index: 1;
    }

    .fe-free-wrap {
      display: flex;
      gap: 8px;
      align-items: center;
      height: 52px;
    }

    .atom-free-fe {
      position: relative;
      width: 32px;
      height: 32px;
      background: #c2410c;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 12px;
      font-weight: 800;
      color: #ffffff;
      box-shadow: inset -1px -1px 3px rgba(0,0,0,0.35), 0 2px 4px rgba(194, 65, 12, 0.4);
    }

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      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      font-size: 12px;
      font-weight: 800;
      color: #1d4ed8;
      padding: 0 4px;
    }

    .arrow-icon {
      font-size: 18px;
      line-height: 1;
    }

    .label-badge {
      font-size: 12px;
      font-weight: 700;
      color: #1e293b;
      margin-top: 2px;
    }

    .sample-block {
      width: 54px;
      height: 52px;
      border-radius: 8px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      color: #ffffff;
      font-weight: 800;
      font-size: 12px;
      box-shadow: 0 2px 4px rgba(0,0,0,0.15);
    }

    .hematite-bg {
      background: linear-gradient(135deg, #991b1b, #7f1d1d);
    }

    .iron-bg {
      background: linear-gradient(135deg, #334155, #1e293b);
    }

    @media (max-width: 360px) {
      .step-btn {
        font-size: 11px;
        padding: 6px 2px;
      }
      .calc-box {
        font-size: 12px;
        padding: 8px;
      }
      .label-badge {
        font-size: 11px;
      }
    }
  </style>
</head>
<body>

  <div class="main-container">
    <div class="nav-bar">
      <button class="step-btn active" onclick="goToStep(0)">1. Fe₂O₃ Moles</button>
      <button class="step-btn" onclick="goToStep(1)">2. 1 : 2 Ratio</button>
      <button class="step-btn" onclick="goToStep(2)">3. Fe Mass</button>
      <button class="play-btn" id="playToggle" onclick="toggleAutoPlay()" title="Toggle Auto-play">⏸</button>
    </div>

    <div class="stage">
      
      <div class="step-panel active" id="step0">
        <div class="step-header">
          <span class="flow-step-tag">Step 1 • Mass to Moles</span>
          <span class="step-indicator">1 of 3</span>
        </div>
        <div class="calc-box">
          <div id="k-molar-mass" class="fallback-formula">
            M<sub>r</sub>(Fe₂O₃) = (2 × 55.8) + (3 × 16.0) = <strong>159.6 g/mol</strong>
          </div>
          <div id="k-moles-calc" class="fallback-formula">
            n(Fe₂O₃) = 320 g ÷ 159.6 g/mol = <strong class="highlight">2.005 mol</strong>
          </div>
        </div>
        <div class="visual-row">
          <div class="cluster">
            <div class="sample-block hematite-bg">
              <div>Fe₂O₃</div>
              <div style="font-size: 10px; font-weight: 600;">320 g</div>
            </div>
            <div class="label-badge">Sample</div>
          </div>
          <div class="arrow-badge">
            <span>÷ 159.6</span>
            <span class="arrow-icon">→</span>
          </div>
          <div class="cluster">
            <div class="fe2o3-unit">
              <div class="atom atom-fe" style="top:4px; left:8px;">Fe</div>
              <div class="atom atom-fe" style="top:4px; left:36px;">Fe</div>
              <div class="atom atom-o" style="top:26px; left:2px;">O</div>
              <div class="atom atom-o" style="top:26px; left:26px;">O</div>
              <div class="atom atom-o" style="top:26px; left:50px;">O</div>
            </div>
            <div class="label-badge highlight">2.005 mol Fe₂O₃</div>
          </div>
        </div>
      </div>

      
      <div class="step-panel" id="step1">
        <div class="step-header">
          <span class="flow-step-tag">Step 2 • Stoichiometric Ratio</span>
          <span class="step-indicator">2 of 3</span>
        </div>
        <div class="calc-box">
          <div id="k-ratio-eqn" class="fallback-formula">
            1 Fe₂O₃ ⟶ 2 Fe (Ratio = 2/1)
          </div>
          <div id="k-fe-moles" class="fallback-formula">
            n(Fe) = 2.005 mol × 2 = <strong class="accent">4.010 mol</strong>
          </div>
        </div>
        <div class="visual-row">
          <div class="cluster">
            <div class="fe2o3-unit">
              <div class="atom atom-fe" style="top:4px; left:8px;">Fe</div>
              <div class="atom atom-fe" style="top:4px; left:36px;">Fe</div>
              <div class="atom atom-o" style="top:26px; left:2px;">O</div>
              <div class="atom atom-o" style="top:26px; left:26px;">O</div>
              <div class="atom atom-o" style="top:26px; left:50px;">O</div>
            </div>
            <div class="label-badge">1 unit (2.005 mol)</div>
          </div>
          <div class="arrow-badge">
            <span>× 2 (Ratio)</span>
            <span class="arrow-icon">→</span>
          </div>
          <div class="cluster">
            <div class="fe-free-wrap">
              <div class="atom-free-fe">Fe</div>
              <div class="atom-free-fe">Fe</div>
            </div>
            <div class="label-badge accent">2 Fe (4.010 mol)</div>
          </div>
        </div>
      </div>

      
      <div class="step-panel" id="step2">
        <div class="step-header">
          <span class="flow-step-tag">Step 3 • Moles to Mass</span>
          <span class="step-indicator">3 of 3</span>
        </div>
        <div class="calc-box">
          <div id="k-mass-formula" class="fallback-formula">
            A<sub>r</sub>(Fe) = <strong>55.8 g/mol</strong>
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        <div class="visual-row">
          <div class="cluster">
            <div class="fe-free-wrap">
              <div class="atom-free-fe">Fe</div>
              <div class="atom-free-fe">Fe</div>
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            <div class="label-badge">4.010 mol Fe</div>
          </div>
          <div class="arrow-badge">
            <span>× 55.8</span>
            <span class="arrow-icon">→</span>
          </div>
          <div class="cluster">
            <div class="sample-block iron-bg">
              <div>Fe</div>
              <div style="font-size: 10px; font-weight: 600;">Pure Metal</div>
            </div>
            <div class="label-badge accent">223.8 g Pure Fe</div>
          </div>
        </div>
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      <div class="unit">(grams)</div>
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      <span class="arrow-icon">↓</span>
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      <span>× Molar Mass B</span>
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      <div class="unit">(mol)</div>
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      <span>× ( Coeff B ÷ Coeff A )</span>
      <span>→</span>
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UK-GCSE-SCIENCE-CHEM-CA-04-01

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