Question

The self ionisation constant for pure formic acid, $K=[HCOO ][ HCOO^{-} ]$ has been estimated as $10^{-6}$ at room temperature. The density of formic acid is $1.15 \,g / cm ^{3}$. The percentage of formic acid molecules in pure formic acid are converted to formate ion:

• A. $0.002 \%$
• B. $0.004 \%$
• C. $0.006 \%$
• D. $0.008 \%$

Answer 1

Answer: (B)

Given density of formic acid $=1.15 \,g / cm ^{3}$
$\therefore $ Weight of formic acid in 1 litre solution $=1.15 \times 10^{3} \,g$
Thus, $[ HCOOH ]=\frac{1.15 \times 10^{3}}{46}=25 \,M$
Since in case of auto-ionisation
$\left[ HCOOH _{2}^{+}\right]=[ HCOO^- ]$ and $[ HCOO^- ]\left[ HCOOH _{2}^{+}\right]=10^{-6}$
$\Rightarrow [ HCOO ^-]=10^{-3}$
Now $\%$ dissociation of $HCOOH$
$=\frac{\left[ HCOO ^{-}\right] \times 100}{[ HCOOH ]}=\frac{10^{-3}}{25} \times 100$
$=0.004 \%$