Complete combustion of 750 g of an organic compound provides 420 g of CO₂ and 210 g of H₂O. The percentage composition of carbon and hydrogen in organic compound is 15.3 and ___________ respectively. (Round off to the Nearest Integer).

Solution

Chemistry ma’am · 4 steps

All the hydrogen ends up in the water, so find how much of that water's mass is hydrogen.

1. Where hydrogen goes. On complete combustion, every hydrogen atom of the compound ends up in H₂O. So the mass of hydrogen in 210 g of water equals the mass of hydrogen in 750 g of the compound.
2. Hydrogen in water. One mole of H₂O (18 g) contains 2 g of hydrogen. So 210 g of water holds 210 × 2/18 = 23.33 g of hydrogen.

   % H = 2/18 × m_(H₂O)/m_(compound) × 100 = 2/18 × 210/750 × 100 = 3.11

3. Check with carbon. The same method for carbon gives 12/44 × 420/750 × 100 = 15.27, which is the 15.3 quoted in the question. So the method matches.
4. So. The percentage of hydrogen is 3.11, which rounds to 3.

The common slip: 2. Using 1/18 instead of 2/18 forgets that each water molecule has two hydrogen atoms. It gives 1.56%, which rounds to 2.

The idea to keep. For combustion analysis, %C uses the factor 12/44 on CO₂ and %H uses 2/18 on H₂O. Always divide by the mass of the original compound.

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