On igniting a mixture of acetylene [C₂H₂] and oxygen, 200 cm³ of CO₂ is collected at s.t.p. Calculate the volume of acetylene & O₂ at s.t.p. in the original mixture.

[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 acetylene :

$\begin{matrix} \text{CO}$2 & : & \text{C}2 \text{H}_2 & \ 4 & : & 2 \ 200 & : & x \end{matrix}

Therefore,

$\dfrac{2}{4} \times 200 = x \\[0.5em] \Rightarrow x = 100 \text { cm}^3$

Hence, volume of acetylene in the original mixture is 100 cm³.

To calculate the volume of oxygen :

$\begin{matrix} \text{CO}$2 & : & \text{O}2 & \ 4 & : & 5 \ 200 & : & x \end{matrix}

Therefore,

$\dfrac{5}{4} \times 200 = x \\[0.5em] \Rightarrow x = 250 \text { cm}^3$

Hence, volume of oxygen in the original mixture is 250 cm³.