Question

On complete combustion, 0.3 g of an organic compound gave 0.2 g of CO2 and 0.1 g of H2O. The percentage composition of carbon and hydrogen in the compound, respectively, is:

• A. 4.07% and 15.02%
• B. 18.18% and 3.70%
• C. 15.02% and 4.07%
• D. 3.70% and 18.18%

Answer 1

Answer: (B)

18.18% and 3.70%

Answer 2

Percentage of Carbon:
The mass of carbon in 0.2 g of CO$_2$ can be calculated using the molar masses: Molar mass of CO$_2$ = 44 g/mol Molar mass of C = 12 g/mol

Mass of C in 0.2 g CO$_2 = \frac{12}{44} \times 0.2 g = 0.0545g$

Percentage of C $= \frac{Mass of C}{Mass of compound} \times 100 = \frac{0.0545}{0.3} \times 100 = 18.18$

Percentage of Hydrogen:

The mass of hydrogen in 0.1 g of H$_2$O can be calculated using the molar masses: Molar mass of H$_2$O = 18 g/mol Molar mass of H = 1 g/mol (but there are 2 H atoms, thus, 2 g/mol)

Mass of H in 0.1 g H$_2$O $= \frac{2}{18} \times 0.1g = 0.0111g$

Percentage of H $= \frac{\text{Mass of H}}{\text{Mass of compound}} \times 100 = \frac{0.0111}{0.3} \times 100 = 3.70$

Therefore, the percentage composition of carbon and hydrogen is 18.18% and 3.70%, respectively.