Question

$100$ mL of a gas at NTP was heated with tin. Tin got converted into stannous sulphide and hydrogen was left. This hydrogen when passed over hot $CuO$ produced $0.081$ g of water. If the vapour density of the gas is $17$, then the number of atoms it contains in its formula is :

Answer 1

Tin is converted into sulphide and hydrogen is left.

This means that the gas contains $H$ and $S$, say $H_a{S}_b$.
$H_a{S}_b+b{S}_n\to bSnS+\frac{a}{2}{H}_2\stackrel{CuO}{\to }\frac{a}{2}{H}_{2}O$
The reaction suggests that moles of $H_2$ formed = moles of $H_{2}O$ formed and

moles of $H_a{S}_b=\frac{a}{2}$ moles of $H_2$ $=\frac{a}{2}$ moles of $H_{2}O$

$\therefore \frac{100}{22400}=\frac{a}{2}\times \frac{0.081}{18}$\ $⟹a=2$

$\because$ Molar mass of $H_a{S}_b=V.D\times 2=17\times 2=34$
$\therefore 1\times a+32\times b=34$
$1\times 2+32\times b=34$
$b=1$
Thus, gas is $H_{2}S$ and it contains $2H+1S=3$ atoms in its formula.