Question

A gaseous compound is composed of $85.7 \%$ by mass carbon and $14.3 \%$ by mass hydrogen. Its density is $2.28$ $g /$ litre at $300 \,K$ and $1.0\, atm$ pressure. Determine the molecular formula of the compound.

• A. $C _{2} H _{2}$
• B. $C _{2} H _{4}$
• C. $C _{4} H _{8}$
• D. $C _{4} H _{10}$

Answer 1

Answer: (C)

$d=\frac{P M}{R T} \Rightarrow M=\frac{d R T}{P}=\frac{2.28 \times 0.0821 \times 300}{1}$
$=56.15 g / mol$
Atomic ratio $=\frac{85.7}{12}: \frac{14.3}{1}=7.14: 14.3=1: 2$;
$\therefore $ E.F is $CH _{2} ; $ M.F. $=\left( CH _{2}\right) n$
where $n=\frac{56.15}{12+2} \simeq 4 ; $
$\therefore$ M.F. is $C _{4} H _{8}$