A sample of a compound contains 60.0 g C and 5.05 g H. Its molar mass is 78.12 g/mol. What is the compound’s molecular formula?

Convert each mass to moles: 60.0 g C ÷ 12.01 g/mol ≈ 5.00 mol C, and 5.05 g H ÷ 1.008 g/mol ≈ 5.01 mol H. The simplest whole-number ratio is 1:1, so the empirical formula is CH, with a mass of about 13.02 g/mol. Since 78.12 ÷ 13.02 = 6, multiply both subscripts by 6 to get C₆H₆, choice C.

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