A compound, X contains 92.31 % C and 7.69 % H.
RMM of X is 78. Determine the molecular formula.

C6H6

Explanation:

We divide each percentage by the relative atomic mass of the element:

Carbon:  92.31 / 12.011 = 7.68

Hydrogen:  7.69 / 1.008 = 7.63

So the ratio of carbon to hydrogen is 1:1.

Empirical formula is CH  - which has a  MM of 13.019

The RMM is 78 so the molecular formula = 78/13.019 = 6 * CH

So it is C6H6  (Benzene).

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.