Question

A compound with empirical formula $C{H}_{2}O$ has a vapour density of $30$ . Its molecular formula is:

• A. $C{H}_{2}O$
• B. $C_2{H}_4{O}_2$
• C. $C_3{H}_6{O}_3$
• D. $C_6{H}_{12}{O}_6$

Answer 1

Answer: (B)

Use the following formula to find the molecular formula of compound.
(i) Molecular mass $=2\times$ vapour density
(ii) $n=$ molecular mass / empirical formula mass
(iii) Molecular formula $=n\times$ empirical formula
Given: Vapour density of $C{H}_{2}O=30$
$=2\times 30=60$
Empirical formula weight of
$C{H}_{2}O=12+1\times 2+16=30$
$n=\frac{\text{molecular weight}}{\text{empirical formula weight}}$
$=\frac{60}{30}=2$
$\therefore$ Molecular formula $=n\times$ empirical formula
$=2(C{H}_{2}O)$
$=C_2{H}_4{O}_2$