200 cm³ of CO₂ is collected at STP when a mixture of acetylene and oxygen is ignited. Calculate the volume of acetylene and oxygen at STP in the original mixture.

2C₂H₂ (g) + 5O₂ (g) ⟶ 4CO₂ (g)+ 2H₂O (g)

According to Gay 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.} \\ \end{matrix}

4 Vol of CO₂ is collected with 2 Vol. of C₂H₂

So, 200 cm³ CO₂ will be collected with

$=\dfrac{2}{4} \times 200 \\[0.5em] = 100 \text{ cc}$

Similarly, 4 Vol of CO₂ is produced by 5 Vol of O₂

So, 200 cm³ CO₂ will be produced by = $\dfrac{5}{4}$ x 200 = 250 cm³

Hence, acetylene = 100 cm³ and oxygen = 250 cm³