Molecular Mass and the Mole: Quick Revision

A brief note on Molecular Mass and the Mole. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Molecular mass (or formula unit mass) = the sum of the atomic masses in the formula, in u.
• One mole holds $N_A = 6.022 \times 10^{23}$ particles.
• Molar mass in g/mol has the same number as the molecular mass in u.
• The map: grams $\div M$ → moles $\times N_A$ → particles; then times the atoms in a molecule.
• Share of an element = its mass in the formula ÷ the molar mass × $100$.

The formulas at a glance

A worked example

How many atoms are in $11.5\ \text{g}$ of ethanol, $\text{C}_2\text{H}_5\text{OH}$?

1. Molar mass. $2 \times 12\ \text{g/mol} + 6 \times 1\ \text{g/mol} + 16\ \text{g/mol} = 46\ \text{g/mol}$.
2. Moles. $\dfrac{11.5\ \text{g}}{46\ \text{g/mol}} = 0.25\ \text{mol}$.
3. Atoms. Each molecule has $9$ atoms: $0.25 \times 9 \times 6.022 \times 10^{23} = 1.355 \times 10^{24}$.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Using $16\ \text{g/mol}$ for oxygen gas. $\text{O}_2$ is $32\ \text{g/mol}$.
• Stopping at molecules when atoms are asked for.
• Multiplying grams by $N_A$ without changing to moles first.
• Forgetting the bracket when adding up a molar mass.

Check yourself

1. What is the molar mass of calcium hydroxide, $\text{Ca(OH)}_2$?

2. How many moles are in $8.4\ \text{g}$ of nitrogen gas, $\text{N}_2$?

3. What is the mass of $1.5\ \text{mol}$ of methane, $\text{CH}_4$?

4. What share of sodium hydroxide, $\text{NaOH}$, is sodium? Use $\text{Na} = 23$, $\text{O} = 16$ and $\text{H} = 1$.