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A chemist analyzes an unknown compound and finds: (i) the ratio of H atoms to O atoms is 2:1, (ii) the compound contains 40% carbon by mass, (iii) the compound contains only C, H, and O. What is the empirical formula of the compound?

1. CH3O
2. CH2O
3. C2H2O
4. CH3O2

Correct answer: CH2O

Solution

Let empirical formula = Cₓ H_y O_z. Given H:O = 2:1, so y = 2z. Let z=1, y=2. Formula is Cₓ H₂ O. Molar mass = 12x + 2 + 16 = 12x + 18. %C = 12x/(12x+18) = 0.40. So 12x = 4.8x + 7.2 => 7.2x = 7.2 => x = 1. Empirical formula = CH2O.

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