Let the hydrocarbon be an alkane with general formula CnH2n+2.
Molar mass =14n+2=72⇒14n=70⇒n=5
The molecular formula is C5H12.
The structural isomers of C5H12 are:
n-pentane: CH3−CH2−CH2−CH2−CH3 (2 primary carbons)
2-methylbutane: CH3−CH(CH3)−CH2−CH3 (3 primary carbons)
2,2-dimethylpropane: C(CH3)4 (4 primary carbons)
The hydrocarbon with exactly three primary carbon atoms is 2-methylbutane.
Answer: 2-Methylbutane