Complete combustion of 1.80 g of an oxygen containing compound (CₓH y O z ) gave 2.64 g of CO₂ and 1.08 g of H₂O. The percentage of oxygen in the organic compound is :

Solution

Chemistry ma’am · 4 steps

Oxygen is never measured directly: find carbon and hydrogen, then oxygen is whatever mass is left.

1. Carbon from CO2. All the carbon ends up in CO₂, and 12 g of every 44 g of CO₂ is carbon. So mass of C = 2.64 × 12/44 = 0.72 g.
2. Hydrogen from water. All the hydrogen ends up in H₂O, and 2 g of every 18 g of water is hydrogen. So mass of H = 1.08 × 2/18 = 0.12 g.
3. Oxygen by difference. The compound has only C, H and O, so oxygen is the remaining mass: 1.80 - (0.72 + 0.12) = 0.96 g.

   % O = 0.96/1.80 × 100 = 53.33

4. So. The compound is 40% C, 6.67% H and 53.33% O, which is option (d). The three add up to 100, a quick check that nothing was missed.

The idea to keep. In combustion analysis, use 12/44 for carbon from CO₂ and 2/18 for hydrogen from H₂O. Oxygen always comes by subtracting from the sample mass.

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