5.6 Empirical and Molecular Formulas

Notes

The empirical formula is the simplest whole-number ratio of atoms. The molecular formula is the actual number of atoms in a molecule.

• Glucose: molecular formula $\text{C}_6\text{H}_{12}\text{O}_6$, empirical formula $\text{CH}_2\text{O}$.
• Finding an empirical formula: (1) assume 100 g so percents become grams, (2) convert grams to moles, (3) divide each by the smallest number of moles, (4) multiply to get whole numbers if needed (for example, 1.5 becomes 3 when you multiply by 2).
• Finding a molecular formula: $\displaystyle n = \frac{\text{molar mass}}{\text{empirical formula mass}}$, then multiply all subscripts by $n$.
• Example: 40.0% C, 6.7% H, 53.3% O gives $\displaystyle \frac{40.0}{12.01} = 3.33$, $\displaystyle \frac{6.7}{1.01} = 6.63$, $\displaystyle \frac{53.3}{16.00} = 3.33$; ratio 1 : 2 : 1, so $\text{CH}_2\text{O}$.