Question

$120g$ of an organic compound that contains only carbon and hydrogen gives $330g$ of $C{O}_2$ and $270g$ of water on complete combustion. The percentage of carbon and hydrogen, respectively are

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

Answer 1

The correct option is B $75$ and $25$

During the complete combustion of any oganic compound, $C{O}_2$ and $H_{2}O$ are produced.
The mass of $C$ in organic compound will be same as the mass of $C$ in $C{O}_2$.
Similarily, the mass of $H$ in organic compound will be same as the mass of $H$ in $H_{2}O$.

Mass of organic compound $=120g$

Mass of $C{O}_2=330g$

Moles of $C{O}_2=\frac{330}{44}=7.5$

Mass of carbon $=7.5\times 12=90g$

Percentage of $C=\frac{90\times 100}{120}=75\%$

Mass of $H_{2}O=270g$

Moles of $H_{2}O=\frac{270}{18}=15$

Mass of hydrogen $=15\times 2=30g$

Percentage of $H=\frac{30\times 100}{120}=25\%$