Question

A mixture of formic acid and oxalic acid is heated with conc. $H_{2}S{O}_4$. The gas produced is collected and treated with alkaline solution, whereby the volume decreases by $\frac{1}{6}$th. The molar ratio of the two acids (formic acid/oxalic acid) is :

• A. $4:1$
• B. $1:4$
• C. $2:1$
• D. $1:2$

Answer 1

Answer: (A)

$4:1$

$HCOOH\underset{-H_{2}O}{\overset{H_{2}S{O}_4}{\to }}CO$

$(COOH{)}_2\underset{-H_{2}O}{\overset{H_{2}S{O}_4}{\to }}CO+C{O}_2$
The volume decreases by 1/6th. The decrease in the volume is due to $C{O}_2$. The volume of carbon dioxide is one sixth the total volume (sum of the volumes of CO and $C{O}_2$).
Hence, $\frac{V_{C{O}_2}}{V_{CO}+V_{C{O}_2}}=\frac{1}{6}=\frac{1}{5+1}$.

The ratio of $V_{CO}$ to $V_{C{O}_2}$ is 5:1.
For every L of $C{O}_2$ produced from oxalic acid, 1 L of CO is produced from oxalic acid. CO produced from formic acid is $5-1=4$ L.
Thus, the ratio of formic acid to oxalic acid is 4:1.