0.5 g of an organic compound on combustion gave 1.46 g of CO₂ and 0.9 g of H₂ O. The percentage of carbon in the compound is _______________. (Nearest integer)
[Given : Molar mass (in g mol⁻¹) C: 12, H: 1, O: 16 ]

Solution

Chemistry ma’am · 3 steps

All the carbon ends up in CO₂, so take 12/44 of its mass and compare with the sample.

1. Where carbon goes. On combustion every carbon atom in the compound becomes CO₂. In 44 g of CO₂ there are 12 g of carbon, so carbon is 12/44 of the CO₂ mass. The water only tells you about hydrogen, so you do not need it here.

   % C = 12/44 × (mass of CO₂)/(mass of compound) × 100

2. Put in numbers. Mass of carbon = 12/44 × 1.46 g = 0.398 g. Then % C = 0.398/0.5 × 100 = 79.64.
3. So. The carbon content is 79.64%, which rounds to 80.

The common slip: 20. Using the water mass with 2/18 gives the hydrogen percentage, 20, not the carbon percentage.

The idea to keep. For carbon, use only the CO₂ mass with the factor 12/44. For hydrogen, use only the water mass with 2/18. Divide by the sample mass at the end.

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