Organic Chemistry Basics 2 Question 1
1. $25 \mathrm{~g}$ of an unknown hydrocarbon upon burning produces $88 \mathrm{~g}$ of $\mathrm{CO}{2}$ and $9 \mathrm{~g}$ of $\mathrm{H}{2} \mathrm{O}$. This unknown hydrocarbon contains
(2019 Main, 12 April II) (a) $20 \mathrm{~g}$ of carbon and $5 \mathrm{~g}$ of hydrogen
(b) $22 \mathrm{~g}$ of carbon and $3 \mathrm{~g}$ of hydrogen
(c) $24 \mathrm{~g}$ of carbon and $1 \mathrm{~g}$ of hydrogen
(d) $18 \mathrm{~g}$ of carbon and $7 \mathrm{~g}$ of hydrogen
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Answer:
Correct Answer: 1. (c)
Solution:
- Hydrocarbon containing $\mathrm{C}$ and $\mathrm{H}$ upon burning produces $\mathrm{CO}_{2}$ and water vapour respectively. The equation is represented as
$$ \begin{aligned} \mathrm{C}{x} \mathrm{H}{y} & +(x+y / 4) \mathrm{O}{2} \longrightarrow x \mathrm{CO}{2}+(y / 2) \mathrm{H}{2} \mathrm{O} \ \text { Mass of carbon } & =\frac{12}{44} \times \text { mass of } \mathrm{CO}{2} \ & =\frac{12}{44} \times 88 \mathrm{~g}=24 \mathrm{~g} \end{aligned} $$
Mass of hydrogen $=\frac{2}{18} \times$ mass of $\mathrm{H}_{2} \mathrm{O}$
$$ =\frac{2}{18} \times 9=1 \mathrm{~g} $$
So, the unknown hydrocarbon contains $24 \mathrm{~g}$ of carbon and $1 \mathrm{~g}$ of hydrogen.
