An organic compound contains 40% carbon, 6.67% hydrogen, and the remainder is oxygen by mass. If the molar mass of the compound is 180 g/mol, what is its molecular formula?

Model Answer & Options

Source: Extra Practice

CH2O

C2H4O2

C6H12O6

C3H6O3

Explanation

Step 1: Find moles of each element in 100g. C: 40/12 = 3.33 mol; H: 6.67/1 = 6.67 mol; O: (100 - 40 - 6.67) = 53.33/16 = 3.33 mol. Step 2: Simplest molar ratio is C:H:O = 3.33/3.33 : 6.67/3.33 : 3.33/3.33 = 1:2:1. Thus, the empirical formula is CH2O. Step 3: Empirical formula mass = 12 + 2(1) + 16 = 30 g/mol. Step 4: n = (Molar Mass / Empirical Mass) = 180/30 = 6. Step 5: Molecular formula = (CH2O) × 6 = C6H12O6. CH2O is the empirical formula, not the molecular formula.

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