Acetylene [C₂H₂] burns in air forming carbon dioxide and water vapour. Calculate the volume of air required to completely burn 50 cm³ of acetylene. [Assume air contains 20% oxygen].

[By Lussac's law]

$\begin{matrix} 2\text{C}$2 \text{H}2 & + & 5\text{O}2 & \longrightarrow & 4\text{CO}2 & + & 2\text{H}_2\text{O} \ 2 \text{ vol.} & : & 5 \text{ vol.} & \longrightarrow & 4\text{ vol.} & : & 2\text{ vol.} \end{matrix}

To calculate the volume of air required,

$\begin{matrix} \text{C}$2 \text{H}2 & : & \text{O}_2 & \ 2 & : & 5 \ 50 & : & x \end{matrix}

Therefore, volume of oxygen (x),

$\dfrac{5}{2} \times 50 = x \\[0.5em] \Rightarrow x = 125 \text{ cm}^3$

When oxygen is 20% then air is 100%
Therefore when, oxygen is 125 cm³ then air is

$\dfrac{100}{20} \times 125 = x \\[0.5em] \Rightarrow x = 625 \text{ cm}^3$

Therefore, volume of air required is 625 cm³.