Question

$10mL$ of gaseous organic compound contain $C,H$ and $O$ only was mixed with $100mL$ of $O_2$ and exploded under identical conditions and then cooled. The volume left after cooling was $90mL$. On treatment with $KOH$ a contraction of $20mL$ was observed. If vapour density of compound is $23$ derive molecular formula of the compound.

Answer 1

• On treatment with $KOH$ a contraction of $20ml$ was observed.
• The contraction of $20ml$ is due to the absorption of carbon dioxide. Thus $10ml$ of compound gives $20ml$ of carbon dioxide.
• The volume of carbon dioxide is twice the volume of the compound.
  

[Volume is $\propto$ number of moles]

Hence, $1$ mole of the compound contains $2$ moles of C atoms.

Also, Molecular weight$=2\times vapourdensity=2\times 23=46\frac{gm}{mol}$. The atomic weight of $C$ is $12\frac{gm}{mol}$.

Two moles of $C$ will weigh $2\times 12=24gm.$

Out of $46gm$ for $1$ mole of compound, $24gm$ corresponds to $2$ moles of $C$.

The remaining $46-24=22gm$ corresponds to oxygen and hydrogen.

The atomic masses of $O$ and $H$ are $16\frac{gm}{mol}$ and $1\frac{gm}{mol}$ respectively.

Hence, $22gm$ can have only one mole of $O$ atoms and $6$ moles of $H$ atoms.

Thus, the molecular formula of the compound is $C_2{H}_{6}O$.

$x+y+z=2+6+1=9$