What is the molecular formula of a hydrocarbon with molar mass 98 g mol^-1?

Molar mass of C = 12 g/mol
Molar mass of H = 1 g/mol

The formula of a hydrocarbon is CₙHₘ which is about (CH₂)ₙ.

Hence, the approximate molar mass of the hydrocarbon in g/mol :
(12 + 1×2) ≈ 98
14n ≈ 98
n = 7

Hence, the molecular formula of the hydrocarbon = C₇H₁₄

This is, of course, either an alkene or a cycloalkane. I ll vote for methylcyclohexane.