116 g of a substance upon dissociation reaction, yields 7.5 g of hydrogen, 60 g of oxygen and 48.5 g of carbon. Given that the atomic masses of H, O and C are 1, 16 and 12, respectively. The data agrees with how many formulae of the following?
A. CH₃COOH, B. HCHO, C. CH₃OOCH₃, D. CH₃CHO

Solution

Chemistry ma’am · 3 steps

Turn the masses into a mole ratio, then keep only the formulae that reduce to that same simplest ratio.

1. Masses to moles. Divide each mass by its atomic mass. H: 7.5/1 = 7.5 mol, O: 60/16 = 3.75 mol, C: 48.5/12 ≈ 4.04 mol. The masses add to 116 g, so nothing is missing.
2. Simplest ratio. Divide each by the smallest, 3.75. C : H : O = 1.08 : 2 : 1, which rounds to 1 : 2 : 1. The empirical formula is CH₂O.

   Compound	Molecular formula	Simplest C : H : O	Fits CH₂O?
   A. CH₃COOH	C₂H₄O₂	1 : 2 : 1	yes
   B. HCHO	CH₂O	1 : 2 : 1	yes
   C. CH₃OOCH₃	C₂H₆O₂	1 : 3 : 1	no
   D. CH₃CHO	C₂H₄O	2 : 4 : 1	no

3. So. Only CH₃COOH and HCHO share the empirical formula CH₂O. The data agrees with 2 formulae.

The common slip: 3. Counting CH₃OOCH₃ because it has two C and two O, like acetic acid, gives 3. It has six H, so its simplest ratio is CH₃O, not CH₂O.

The idea to keep. Mass data alone can only fix the empirical formula. Any compound whose molecular formula reduces to that simplest ratio fits the data equally well.

Answer from the NTA final answer key. Solution by PrepWiser.