Calculate the volume of air at STP, required to convert 300 mL of sulphur dioxide to sulphur trioxide. Air contains 21% oxygen by volume.

Chemical equation for conversion of sulphur dioxide to sulphur trioxide can be written as:
${\mathrm{SO}}_2+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{SO}}_3$
According to the balanced chemical equation, to convert one mole of SO₂ to SO₃, half moles of oxygen is needed.
As we know that at STP one mole of any gas occupies 22,400 mL of volume.
Volume of oxygen needed to convert 22,400 mL of SO₂ to SO₃ = $\frac{22400}{2}\mathrm{mL}$
Volume of oxygen needed to convert 300 mL of SO₂ to SO₃ = $\frac{22400}{2}\times \frac{300}{22400}\mathrm{mL}=150\mathrm{ml}$
Now, air contains 21% of oxygen.
Volume of air containing 150 mL of oxygen = $150\times \frac{100}{21}=714.28\mathrm{mL}$