1.3 Elemental Composition of Pure Substances

Empirical formulas from mass data

The empirical formula shows the simplest whole-number ratio of atoms in a compound. Given mass percent composition, convert each element's mass to moles, then divide all mole values by the smallest one to find the ratio. Round to whole numbers (or find a common multiplier) to get the empirical formula.

Mass percent composition

Percent composition is the mass of each element divided by the total molar mass of the compound, times 100%. This can be calculated from a chemical formula or measured experimentally through techniques like combustion analysis, which determines the mass of C and H (as CO2 and H2O) produced when a sample burns.

Molecular vs. empirical formula

The molecular formula is a whole-number multiple of the empirical formula, reflecting the actual number of atoms in a molecule. To find it, divide the compound's actual molar mass by the empirical formula's molar mass to get the multiplier, then multiply each subscript in the empirical formula by that number.

Key terms

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound.

Molecular formula

The actual number of atoms of each element in a molecule; a whole-number multiple of the empirical formula.

Percent composition

Mass of an element divided by total molar mass of the compound, times 100%.

Combustion analysis

A technique that burns a sample and measures CO2 and H2O produced to determine moles of C and H in the original compound.

Steps to find empirical formula from % composition

Convert % to grams (assume 100 g sample) → convert grams to moles → divide by smallest mole value → round to whole-number ratio.

Try an AP-style question

What is the mass percent of carbon in CO₂ (C = 12.0, O = 16.0)?

1. 72.7%
2. 27.3%
3. 50.0%
4. 33.3%