Question

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

• A. $ CH_{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 $ CH_{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\,(CH_{2}O) $
$= C _{2} H _{4} O _{2}$