Identify the correct IUPAC name of hydrocarbon ( x ) containing...

Let the hydrocarbon be an alkane with general formula $C_nH_{2n+2}$.

Molar mass $= 14n + 2 = 72 \Rightarrow 14n = 70 \Rightarrow n = 5$

The molecular formula is $C_5H_{12}$.

The structural isomers of $C_5H_{12}$ are:

n-pentane: $CH_3-CH_2-CH_2-CH_2-CH_3$ (2 primary carbons)

2-methylbutane: $CH_3-CH(CH_3)-CH_2-CH_3$ (3 primary carbons)

2,2-dimethylpropane: $C(CH_3)_4$ (4 primary carbons)

The hydrocarbon with exactly three primary carbon atoms is 2-methylbutane.

Answer: 2-Methylbutane