Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Some pure substances are made of separate molecules (H₂O, CO₂). Others are atoms or ions held together in a fixed ratio, described by a formula unit (NaCl, SiO₂).
- Law of definite proportions: every pure sample of a compound has the same mass ratio of its elements. Pure water is always 11.19% H and 88.81% O by mass.
- Different compounds of the same elements have different ratios (H₂O₂ is 5.93% H).
- The empirical formula is the lowest whole-number ratio of atoms. C₆H₁₂O₆ → CH₂O.
- Mass composition fixes the empirical formula only. Compounds that share an empirical formula (C₂H₂ and C₆H₆) have the same percentages.
Key relationships
| Relationship | Use it to |
|---|---|
| % element = (atoms in formula × atomic mass) ÷ M × 100 | find percentage composition from a formula |
| mass of element = mass of sample × (% element ÷ 100) | predict an element’s mass in any pure sample |
| n = m ÷ atomic mass, for each element | turn element masses into moles |
| mole ratio = each n ÷ smallest n | find the atom ratio |
Whole-number fixes: .50 → × 2; .33 or .67 → × 3; .25 or .75 → × 4; .20, .40, .60, .80 → × 5.
Assumptions behind the numbers
- The sample is pure. A mixture can have any composition.
- In a “metal + oxygen” experiment, the reaction is complete and the mass gain is all oxygen.
- Masses are accurate enough that each ratio lands within a few hundredths of a simple fraction.
Mistakes to avoid
- Comparing percentages as if they were atom ratios. Convert to moles first.
- Rounding 1.5 to 2 or 1.33 to 1. Multiply the whole ratio instead.
- Dividing by atomic number. Use atomic mass.
- Using the product’s mass as the oxygen mass. Oxygen mass = product mass − metal mass.
- Calling NaCl a molecule. It is a formula unit in a lattice.
- Rounding mole values early. Keep four significant figures until the ratio step.
Quick self-check
- What is the empirical formula of N₂O₄? (NO₂)
- What percentage of CO₂ is carbon by mass? (27.29%)
- A compound is 30.45% N and 69.55% O by mass. What is its empirical formula? (NO₂)
- A mole ratio comes out as P 1.000 : O 2.500. What is the empirical formula? (P₂O₅)
Next: practice questions.