The equation,

4NH₃ + 5O₂ ⟶ 4NO + 6H₂O

represents the catalytic oxidation of ammonia. If 100 cm³ of ammonia is used, calculate the volume of oxygen required to oxidise the ammonia completely.

[By Lussac's law]

$\begin{matrix} 4\text{NH}$3 & + & 5\text{O}2 &\longrightarrow & 4\text{NO} & + & 6\text{H}_2\text{O} \ 4 \text{ vol.} & : & 5 \text{ vol.} & \longrightarrow & 4\text{ vol.} \end{matrix}

To calculate the volume of oxygen required :

$\begin{matrix}\text{NH}$3 & : & \text{O}2 \ 4 \text{ vol.} & : & 5 \text{ vol.} \ 100 \text{ cm.}^3 & : & \text{x} \end{matrix}

$\therefore x = \dfrac{5}{4} \times 100 = 125 \text{ cm}^3$

Hence, volume of oxygen required = 125 cm³