Empirical formula of C7H9O and a molar mass of 215.0 g/mol. Determine molecular formula?

Atomic mass of C: 12.0 AMU, therefore 12.0 g/mol.
Atomic mass of H: 1.0 g/mol
Atomic mass of O: 16.0 g/mol

Atomic mass of empirical formula = 7 * 12.0 + 9 * 1.0 + 1 * 16.0 = 109 g/mol

215 g/mol divided by 109 g/mol = 1.97 or ~2. So, you chalk the imprecision up to slight variations in the isotopic ratios (and rounding errors) and say the factor is TWO.

Then you double every number in the empirical formula and find the answer.

Answer: C)