lesson
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If you burn 2.43ย g of shiny magnesium ribbon in air, it transforms into 4.03ย g of a brittle white powder.
That extra 1.60ย g came directly from oxygen in the air, but does that mean magnesium and oxygen combined in a 1-to-1 atom ratio, or did one element contribute more atoms than the other?
Counting Atoms by Weighing
An empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
Because every element has a different atomic mass, equal grams do not mean equal numbers of atoms โ a single magnesium atom weighs roughly 1.5 times as much as an oxygen atom.
๐A visual comparison card showing atomic weights. On the left, an atom of Magnesium labeled 'Mg (Ar = 24.3)' with a visual size/mass circle. On the right, an atom of Oxygen labeled 'O (Ar = 16.0)'. Below them, a balance scale: 24.3 g of Mg contains 6.02 x 10^23 atoms (1 mole), while 24.3 g of O contains ~1.5 moles of atoms. Color scheme: slate blue (#1e2945) header, soft gray card background (#f8f9fa), vibrant cyan (#00b4d8) and coral (#ff6b6b) atom spheres with clean minimal borders.
To count atoms, we convert mass into moles using the formula n=Arโmโ, where m is the mass in grams and Arโ is the relative atomic mass found on the periodic table.
In 1799, French chemist Joseph Proust proved the Law of Definite Proportions by showing that chemical compounds always contain elements in fixed mass ratios, allowing scientists to deduce formulas from reacting masses.
So how do we turn raw lab measurements into a clean chemical formula every single time?
The 4-Step Empirical Formula Method
We follow a consistent four-step sequence to determine any empirical formula from laboratory data.
๐A responsive 4-step horizontal flowchart showing the empirical formula pipeline. Step 1: 'Mass (g)' with a balance icon. Step 2: 'Moles (mol)' with formula 'n = mass / Ar'. Step 3: 'Divide by Smallest' with division symbol icon. Step 4: 'Whole Number Ratio' resulting in final empirical formula box (e.g. 'AxBy'). Subtle arrows connect each step with clean badges 1, 2, 3, 4. Light theme (#f8f9fa), dark slate text (#1e2945), sky blue highlighting (#0284c7).
Let's apply this four-step roadmap to our magnesium ribbon experiment.