(i) Oxygen oxidises ethyne to produce 8.4 litres of carbon dioxide at STP. Calculate the volume of ethyne used in this reaction.

2C₂H₂ + 5O₂ ⟶ 4CO₂ + 2H₂O

(ii) A₂B is the empirical formula of a compound which has vapour density 25. If atomic weight of A and B are 10 and 5 respectively. Find it's molecular formula.

(i) [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 ethyne gas

$\begin{matrix}\text{CO}$2 & : & \text{C}2\text{H}_2 \ 4 & : & 2 \ 8.4 & : & x \end{matrix}

$\therefore x = \dfrac{2}{4} \times 8.4 = 4.2 \text{ L}$

Hence, volume of ethyne gas required = 4.2 L.

(ii) Empirical formula is A₂B

Empirical formula weight = 2(10) + 5 = 25

Vapour density (V.D.) = 25

Molecular weight = 2 x V.D. = 2 x 25

$\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{2 \times 25}{25} = 2$

∴ Molecular formula = n[E.F.] = 2[X₂Y] = A₄B₂