Empirical And Molecular Formula

Question

A hydrocarbon contains 92.3% carbon by mass and has a molar mass of 78 g/mol. What is its molecular formula?

RankGuru · Accepted Answer

To find the molecular formula, first determine the empirical formula from percentage composition, then scale it using the molar mass. Carbon is 92.3%, so hydrogen is $100 - 92.3 = 7.7\%$. Moles ratio: C $= 92.3/12 = 7.69$, H $= 7.7/1 = 7.7$. Dividing by the smaller value: C $\approx 1$, H $\approx 1$. Empirical formula is $\text{CH}$ with empirical mass $= 13$ g/mol. Multiplying factor $n = 78/13 = 6$. Therefore molecular formula is $\text{C}_6\text{H}_6$ (benzene). Option $\text{CH}$ is the empirical formula only, not the molecular formula with molar mass 78. Option $\text{C}_2\text{H}_2$ has molar mass 26, inconsistent with 78 g/mol ($n$ would be 3, giving $\text{C}_6\text{H}_6$, but $\text{C}_2\text{H}_2$ itself is wrong). Option $\text{C}_3\text{H}_4$ has molar mass 40, which does not divide 78 to give an integer. This is a classic NCERT empirical-to-molecular formula problem. Plausibility check: $\text{C}_6\text{H}_6$ has molar mass $6(12)+6(1)=78$ g/mol, confirming the answer.

Empirical And Molecular Formula

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