Question

(a) What is the volume occupied by the following gases at STP?
(i) 48 g of oxygen.
(ii) 16 g of sulphur dioxide
(b) Determine the molecular mass of a gas if 5 g of it occupies a volume of 4 litres at STP.

Answer 1

(a)
(i) Number of moles in 48 g of oxygen = $\frac{48}{32}=\frac{3}{2}$
At STP, one mole of oxygen occupies 22.4 litres of volume.
Volume occupied by $\frac{3}{2}$ moles of oxygen at STP = $\left(22.4\times \frac{3}{2}\right)\mathrm{L}=33.6\mathrm{L}$

(ii) Number of moles in 16 g of sulphur dioxide = $\frac{16}{64}=\frac{1}{4}$
At STP, one mole of sulphur dioxide occupies 22.4 litres of volume.
Volume occupied by $\frac{1}{4}$ mole of sulphur dioxide = $\left(22.4\times \frac{16}{64}\right)\mathrm{L}=5.6\mathrm{L}$

(b) Let M be the molecular mass of the gas.
Number of moles in 5 g of the gas = $\frac{5}{M}$
At STP, one mole of a gas occupies 22.4 litres of volume.
Volume occupied by $\frac{5}{M}$ moles of gas = $\left(22.4\times \frac{5}{M}\right)\mathrm{L}$
It is given that volume of gas is four litres.
$$\begin{gathered} 4\mathrm{L}=\left(22.4\times \frac{5}{M}\right)\mathrm{L} \\ M\mathit{}=28 \end{gathered}$$
So, molecular mass of gas is 28 g.