An organic compound gives 0.220 g of CO₂ and 0.126 g of H₂ O on complete combustion. If the % of carbon is 24 then the % of hydrogen is __________ × 10⁻¹. ( Nearest integer)

Solution

Chemistry ma’am · 4 steps

The sample's mass is not given, so let the carbon percentage tell you how much compound was burnt.

1. Carbon from CO2. Each 44 g of CO₂ holds 12 g of carbon. So the carbon burnt is 0.220 × 12/44 = 0.060 g.
2. Mass of compound. This 0.060 g is 24% of the sample. So the sample weighed 0.060/0.24 = 0.25 g.
3. Hydrogen from water. Each 18 g of H₂O holds 2 g of hydrogen. So the hydrogen is 0.126 × 2/18 = 0.014 g.

   % H = 0.014/0.25 × 100 = 5.6

4. So. The compound has 5.6% hydrogen, which is 56 × 10⁻¹. The answer is 56.

The common slip: 28. Taking hydrogen as 1/18 of the water forgets that each H₂O has two H atoms, and gives 2.8%, that is 28.

The idea to keep. In combustion analysis, carbon is 12/44 of the CO₂ and hydrogen is 2/18 of the H₂O. If the sample mass is missing, get it from any percentage you are given.

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