If 6 litres of hydrogen and 4 litres of chlorine are mixed and exploded and if water is added to the gases formed, find the volume of the residual gas.

[By Lussac's law]

$\begin{matrix} \text{H}$2 & + & \text{Cl}2 &\longrightarrow & 2\text{HCl} \ 1 \text{ vol.} & : & 1 \text{ vol.} & \longrightarrow & 2\text{ vol.} \end{matrix}

To calculate the amount of hydrogen used:

$\begin{matrix}\text{Cl}$2 & : & \text{H}2 \ 1 \text{ vol.} & : & 1 \text{ vol.} \ 4 \text{ lit.} & : & \text{x} \end{matrix}

∴

$x = \dfrac{1}{1} \times 4 = 4 \text{ lit.}$

Remaining hydrogen = 6 - 4 = 2 lit.

To calculate the amount of HCl :

$\begin{matrix}\text{Cl}_2 & : & \text{HCl} \ 1 \text{ vol.} & : & 2 \text{ vol.} \ 4 \text{ lit.} & : & \text{x} \end{matrix}$

∴

$x = \dfrac{2}{1} \times 4 = 8 \text{ lit.}$

Hence, after reaction 8 lit of HCl is formed which dissolves in water and 2 lit of hydrogen is left.

∴ 2 lits. of hydrogen is left.