Empirical vs Molecular Formula Cheat Sheet

Key Facts

• An empirical formula shows the simplest whole-number atom ratio, such as $\mathrm{CH_2O}$ for glucose.
• A molecular formula shows the actual number of atoms in one molecule, such as $\mathrm{C_6H_{12}O_6}$ for glucose.
• To convert mass to moles, use $n = \frac{m}{M}$, where $n$ is moles, $m$ is mass in grams, and $M$ is molar mass in $\mathrm{g\ mol^{-1}}$.
• For percent composition problems, assume a $100\ \mathrm{g}$ sample so each percent becomes grams, such as $40.0\%$ carbon becoming $40.0\ \mathrm{g}$ carbon.
• After converting each element to moles, divide all mole amounts by the smallest mole amount to get the simplest ratio.
• If a mole ratio contains decimals near common fractions, multiply all ratios by the same integer, such as $2$ for $0.5$, $3$ for $0.333$, or $4$ for $0.25$.
• The molecular formula multiplier is $n = \frac{\text{molecular molar mass}}{\text{empirical formula mass}}$, and the molecular formula equals $n$ times every subscript in the empirical formula.
• The percent composition of an element is $\%\text{ element} = \frac{\text{mass of element in compound}}{\text{molar mass of compound}} \times 100\%$.

Vocabulary

Empirical formula

The formula that gives the simplest whole-number ratio of elements in a compound.

Molecular formula

The formula that gives the actual number of atoms of each element in one molecule of a compound.

Molar mass

The mass of one mole of a substance, usually measured in $\mathrm{g\ mol^{-1}}$.

Percent composition

The percent by mass of each element in a compound.

Mole ratio

A ratio comparing the amounts in moles of elements or substances in a formula or reaction.

Formula mass

The total mass of all atoms represented in a chemical formula, found by adding the atomic masses of each atom.