Percent Composition and Hydrate Formula Worked Examples Cheat Sheet

Key Facts

• Molar mass is the sum of the atomic masses in a formula, written in units of $\mathrm{g/mol}$.
• Percent composition by mass is $\%\text{ element} = \frac{\text{mass of element in }1\text{ mol compound}}{\text{molar mass of compound}} \times 100\%$.
• The percent compositions of all elements in a compound should add to about $100\%$, allowing small rounding differences.
• For a hydrate, the mass of water lost during heating is $m_{\text{water}} = m_{\text{hydrate}} - m_{\text{anhydrous salt}}$.
• Moles of anhydrous salt are found with $n_{\text{salt}} = \frac{m_{\text{salt}}}{M_{\text{salt}}}$.
• Moles of water are found with $n_{\text{water}} = \frac{m_{\text{water}}}{18.02\ \mathrm{g/mol}}$.
• The hydrate coefficient $x$ in $\text{salt}\cdot x\mathrm{H_2O}$ comes from the mole ratio $x = \frac{n_{\text{water}}}{n_{\text{salt}}}$, rounded to a small whole number.
• The percent water in a hydrate is $\%\mathrm{H_2O} = \frac{m_{\text{water}}}{m_{\text{hydrate}}} \times 100\%$.

Vocabulary

Percent composition

The percentage by mass of each element in a compound.

Molar mass

The mass of one mole of a substance, found by adding the atomic masses in its chemical formula.

Hydrate

An ionic compound that has a fixed number of water molecules attached within its crystal structure.

Anhydrous salt

The salt that remains after all water of crystallization has been removed from a hydrate.

Water of crystallization

Water molecules that are chemically included in the crystal structure of a hydrate.

Mole ratio

A ratio comparing the moles of substances, used to find formula subscripts or hydrate coefficients.