CAIE IGCSE Chemistry Reacting Masses Volumes and Concentrations

Practise multi-step stoichiometry by converting the given quantity to moles, applying an equation ratio and converting to the requested mass, volume or concentration.

Exam points

convert the given mass, gas volume or solution data into moles with consistent units
use coefficients from the balanced equation to obtain the required mole quantity
identify the limiting reactant or excess, then calculate mass, gas volume or concentration

[Maximum number: 3]

The Periodic Table is a method of classifying elements.

$\mathrm{PCl}_{5}$ reacts with lithium fluoride, LiF , to form $\mathrm{LiPF}_{6}$ .

\mathrm{PCl}_{5}+6 \mathrm{LiF} \rightarrow \mathrm{LiPF}_{6}+5 \mathrm{LiCl}

Calculate the mass of LiF needed to form 3.04 g of $\mathrm{LiPF}_{6}$ using the following steps.
- Calculate the number of moles of $\mathrm{LiPF}_{6}$ formed.
$\left[M_{\mathrm{r}}: \operatorname{LiPF}_{6}, 152\right]$

number of moles =

- Deduce the number of moles of LiF needed.
number of moles =
- Calculate the mass of LiF needed.

mass $=\ldots \ldots \ldots ~ g ~$
[3]

mol of $\mathrm{LiPF}_{6}=3.04 / 152=0.02(00)(1)$

mol of LiF $=0.02(00) \times 6=0.12(0)(1)$

mass of LiF $=3.12 \mathrm{~g}$ (1)

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3.3.5—Stoichiometric reacting masses question 1