IB Chemistry HL 1.4.4 Empirical and Molecular Formulas Question Bank

Practise IB Chemistry HL 1.4.4 with questions deriving empirical and molecular formulae from composition data and molar mass.

[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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1.4.4—Empirical and molecular formulas question 1

Question (a)

Question (i)

Question (ii)