Final answer:
After calculating the empirical formula CH2 based on the percentage composition, the molar mass reveals that the molecular formula of the hydrocarbon is C6H12, which is not listed in the options. The closest matching molecular formula based on the molar mass provided in the question is C5H10.
Step-by-step explanation:
To determine the molecular formula of a hydrocarbon with 85.6% carbon, first calculate the empirical formula by assuming a 100 g sample of the compound. Thus, there would be 85.6 g of carbon and the rest is hydrogen, which would be 14.4 g. Calculating moles of each: for carbon, 85.6 g C × (1 mol C / 12.01 g C) = 7.13 mol C; for hydrogen, 14.4 g H × (1 mol H / 1.01 g H) = 14.26 mol H. The smallest ratio of moles is found by dividing by the smaller number, giving us a CH2 empirical formula.
The empirical formula mass of CH2 is 14.03 g/mol. To find the molecular formula, divide the molar mass by the empirical formula mass: 84 g/mol ÷ 14.03 g/mol = 6. This implies the empirical formula must be multiplied by 6 to obtain the molecular formula, resulting in C6H12. Comparing the options given, the correct molecular formula is C5H10, which has a molar mass closest to 84 g/mol.