Question

The equivalent conductance of $\frac{M}{32}$ solution of a weak monobasic acid is $8.0mhosc{m}^2$ and at infinite dilution is $400mhosc{m}^2$. The dissociation constant of this acid is:

• A. $1.25\times {10}^{-4}$
• B. $1.25\times {10}^{-5}$
• C. $1.25\times {10}^{-6}$
• D. $6.25\times {10}^{-4}$

Answer 1

Answer: (B)

$1.25\times {10}^{-5}$

Given, concentration $=\frac{M}{32};{\lambda }_m=8mhosc{m}^2;{\lambda }_m^o=400mhosc{m}^2$

Degree of dissociation $=\frac{{\lambda }_m}{{\lambda }_m^o}=\frac{8}{400}=0.02$

k

$HA\rightleftharpoons H^{+}+A^{+}$

$c$ $-$ $-$

$c(1-\alpha )$ $c\alpha$ $c\alpha$

where $c$ is concentration.

$k=\frac{c{\alpha }^2}{1-\alpha };\alpha <<<1$ (weak acid)

$k=c{\alpha }^2=\frac{1}{32}\times (0.02{)}^2=1.25\times {10}^{-5}M$

Hence, option B is correct.