CAIE A-Level Chemistry 4.1.3 The Ideal Gas Equation

Practise applying pV = nRT to amount, pressure, volume, density and relative molecular mass calculations.

Exam points

convert temperature to K, volume to m3 and pressure to Pa when using R=8.31 J K−1 mol−1
rearrange pV=nRT symbolically before substitution and preserve units through the calculation
combine n=m/Mr or gas density with the equation to determine relative molecular mass

[Maximum number: 3]

The Pauling electronegativity values of elements can be used to predict the chemical properties of compounds.

Use the information in Table 1.1 to answer the following questions.

Table 1.1

At $150^{\circ} \mathrm{C}$ and 103 kPa , all of the compounds listed in Table 1.2 are gases.

Under these conditions, 0.284 g of one of the compounds occupies a volume of $127 \mathrm{~cm}^{3}$ .
Use this information to calculate the $M_{\mathrm{r}}$ of the compound. Hence, identify the compound from those given in Table 1.2.

Show your working.
$M_{\mathrm{r}}=$ identity of compound =

conversion of units

$103000 \mathrm{~Pa} \quad 127 \times 10^{-6} \mathrm{~m}^{3}$

Use of $p V=\left(m / M_{\mathrm{r}}\right) R T$ $M_{\mathrm{r}}=\frac{0.284 \times 8.31 \times 423}{103000 \times 127 \times 10^{-6}}$ $M_{\mathrm{r}}=76.3$ AND compound $=\mathrm{CS}_{2}$

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4.1.3—Ideal gas equation pV = nRT question 1