AP Chemistry / Unit 1: Atomic Structure and Properties
1.3 Elemental Composition of Pure Substances
Notes
A pure compound has a fixed ratio of elements by mass (law of definite proportions).
- Empirical formula: simplest whole-number ratio (from percent composition).
- Percent by mass = (mass of element ÷ mass of compound) × 100.
Practice
The empirical formula of $\mathrm{C_6H_{12}O_6}$:
Divide by 6.
Percent hydrogen by mass in water ($\mathrm{H_2O}$, 18 g/mol):
$\displaystyle \frac{2}{18}\approx11\%$.
A compound has 1 mol C for every 2 mol H. Its empirical formula:
Simplest ratio.
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