1.3 Elemental Composition of Pure Substances

A pure compound always has the same ratio of elements by mass (law of definite proportions). Water is always 11.2% H by mass.

The empirical formula is the simplest whole-number ratio of atoms; the molecular formula is the actual count. Glucose C₆H₁₂O₆ has empirical formula CH₂O.

Percent → formula: assume 100 g, convert each % to moles, divide by the smallest, round to whole numbers.

Worked example: 40.0% C, 6.7% H, 53.3% O → 3.33 : 6.7 : 3.33 mol → 1 : 2 : 1 → CH₂O.