CAIE A-Level Chemistry 2.2 The Mole and the Avogadro Constant

Practise converting among amount, mass and particle number while counting atoms, ions or molecules at the correct level.

Exam points

convert mass to amount using molar mass before multiplying by the Avogadro constant
count atoms, ions or molecules by applying formula subscripts to the amount of substance
use mole ratios in formula or hydrate calculations to identify n or a relative formula mass

Covered syllabus points

2.2.1—Terms: mole

[Maximum number: 2]

Chlorine exists as a diatomic gas, $\mathrm{Cl}_{2}(\mathrm{~g})$ . A sample of $\mathrm{Cl}_{2}(\mathrm{~g})$ was made during a chemical reaction. When measured at 404 kPa and $25^{\circ} \mathrm{C}$ the sample occupied a volume of $20.0 \mathrm{~cm}^{3}$ .

Calculate the number of chlorine atoms in this sample of $\mathrm{Cl}_{2}(\mathrm{~g})$ . You may find it helpful to use your answer to (d)(i).

If you are unable to calculate an answer to (d)(i), use 0.36 g of $\mathrm{Cl}_{2}$ . This is not the correct answer.

number of chlorine atoms =

Method 1 & Method 2

$\mathbf{M} \mathbf{1}=3.263 \times 10^{-3} \times 2$ & $\mathbf{M} \mathbf{1}=\frac{0.23}{71.0} \times 2$ OR $6.53 \times 10^{-3}$ $\mathbf{M} \mathbf{2}=6.02 \times 10^{23} \times \mathbf{M} \mathbf{1}$ $=3.93 \times 10^{21}$ atoms of Cl

$\mathbf{M} \mathbf{2}=6.02 \times 10^{23} \times \mathbf{M} \mathbf{1}$ $=3.90 \times 10^{21}$ atoms of Cl

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Covered syllabus points

2.2 The mole and the Avogadro constant question 1