$A$ hydrocarbon of molecular mass $72 \ g \ mol^{-1}$ gives a single monochloro derivative and two dichloro derivatives on photochlorination. Give the structure of the hydrocarbon.

(N/A) The molecular formula of the hydrocarbon with molecular mass $72 \ g \ mol^{-1}$ is $C_5H_{12}$ $(5 \times 12 + 12 \times 1 = 72)$.
$C_5H_{12}$ has three isomers: $n$-pentane,isopentane,and neopentane.
Neopentane $(2,2-\text{dimethylpropane})$ has all $12$ hydrogen atoms equivalent,so it gives only one monochloro derivative upon photochlorination.
Further chlorination of this monochloro derivative yields two dichloro derivatives: $1,3-\text{dichloro}-2,2-\text{dimethylpropane}$ and $1,1-\text{dichloro}-2,2-\text{dimethylpropane}$.
Thus,the hydrocarbon is neopentane,with the structure: $CH_3-C(CH_3)_2-CH_3$.