Molecular Formula From Empirical Formula

Question

An organic compound contains 40.0% carbon and 6.7% hydrogen with the remainder oxygen, and its molar mass is found to be 180 g per mol; what is the molecular formula of the compound?

RankGuru · Accepted Answer

To find the molecular formula, the empirical formula is determined first and then scaled by the ratio of molar mass to empirical formula mass. The oxygen percentage is 100 - 40.0 - 6.7 = 53.3%. Converting to moles: carbon 40.0/12 = 3.33, hydrogen 6.7/1 = 6.7, oxygen 53.3/16 = 3.33. Dividing by the smallest value gives the ratio 1 : 2 : 1, so the empirical formula is CH2O with empirical formula mass 12 + 2 + 16 = 30 g per mol. The factor n equals molar mass divided by empirical mass = 180 / 30 = 6. Therefore the molecular formula is (CH2O) multiplied by 6, that is C6H12O6, which corresponds to glucose. The option C5H10O5 has a molar mass of 150, not 180, so it is inconsistent with the data. The option CH2O is only the empirical formula with mass 30, far below 180. The option C3H6O3 has mass 90, giving n = 3, which does not match the measured molar mass of 180. This two-step procedure is the NCERT method linking empirical and molecular formulae. Plausibility check: multiplying the empirical mass 30 by 6 gives exactly 180, confirming C6H12O6.

Molecular Formula From Empirical Formula

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