Question

\(120 \;g\) of an organic compound that contains only carbon and hydrogen gives \(330\) \(g\) of \(CO_2\) and \(270\) \(g\) of water on complete combustion. The percentage of carbon and hydrogen, respectively are

• A. 25 and 75
• B. 40 and 60
• C. 60 and 40
• D. 75 and 25

Answer 1

The Correct Option is D

The question involves determining the percentage composition of carbon and hydrogen in an organic compound, based on the by-products of its combustion, namely carbon dioxide \((CO_2)\) and water \((H_2O)\).

1. First, we need to find out the amount of carbon in the carbon dioxide produced. The molar mass of \(CO_2\) is \(44 \; g/mol\) (12 for carbon and 32 for oxygen).
2. The molar mass of carbon, \(C\), is \(12 \; g/mol\).
3. The mass of carbon in \(330 \; g\) of \(CO_2\) is calculated as: \(\frac{12}{44} \times 330 = 90 \; g\).
4. Next, we calculate the amount of hydrogen from the water produced. The molar mass of \(H_2O\) is \(18 \; g/mol\) (2 for hydrogen and 16 for oxygen).
5. The molar mass of hydrogen, \(H_2\), is \(2 \; g/mol\).
6. The mass of hydrogen in \(270 \; g\) of water is: \(\frac{2}{18} \times 270 = 30 \; g\).
7. We are given that the total mass of the compound is \(120 \; g\).
8. The percentage of carbon in the compound is calculated as: \(\frac{90}{120} \times 100 = 75\%\).
9. The percentage of hydrogen in the compound is calculated as: \(\frac{30}{120} \times 100 = 25\%\).

Thus, the percentage composition of carbon and hydrogen in the compound is 75% and 25%, respectively. This is consistent with the answer option provided. Therefore, the correct answer is 75 and 25.