IB Chemistry HL 1.4.4 Empirical and Molecular Formulas Questions

Convert composition data to moles, reduce to an empirical formula, then use molar mass to obtain the molecular formula and check percentage composition.

Exam points

Derive an empirical formula by converting element masses, combustion products or percentage composition to moles and reducing to the simplest whole-number atom ratio.
Determine a molecular or formula-unit composition from an empirical formula and molar mass, and distinguish simplest ratios from actual atoms in a molecule.
Calculate percentage composition by mass from a stated chemical formula, including hydrates and adducts, with appropriate significant figures.

[Maximum number: 4]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

Impurities cause phosphine to ignite spontaneously in air to form an oxide of phosphorus and water.

[ 4 ]

(i)

The oxide formed in the reaction with air contains 43.6% phosphorus by mass. Determine the empirical formula of the oxide, showing your method.

[ 3 ]

$n(\mathrm{P})=\frac{43.6}{30.97}=1.41\ \mathrm{mol}$ $n(\mathrm{O})=\frac{100-43.6}{16.00}=3.53\ \mathrm{mol}$ $\frac{n(\mathrm{O})}{n(\mathrm{P})}=\frac{3.53}{1.41}=2.50$

Empirical formula:
$\mathrm{P}_{2}\mathrm{O}_{5}$

Accept other methods where the working is shown.

(ii)

The molar mass of the oxide is approximately $285 \mathrm{~g} \mathrm{~mol}^{-1}$ .

Determine the molecular formula of the oxide.

[ 1 ]

« $\frac{285}{141.9}=2.00$ , so molecular formula $=2 \times \mathrm{P}_{2} \mathrm{O}_{5}=$ » $\mathrm{P}_{4} \mathrm{O}_{10}$

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IB Chemistry HL 1.4.4 Empirical and Molecular Formulas Questions question 1

Question (a)

Question (i)

Question (ii)