Question

A weak monobasic acid solution has a pH value of $5$. If its molarity is $0.005$ M, then the degree of ionisation of the acid is:

• A. $5\times {10}^{-3}$
• B. $2\times {10}^{-3}$
• C. $5\times {10}^{-2}$
• D. $2\times {10}^{-2}$

Answer 1

The correct option is B $2\times {10}^{-3}$

Given that a weak monobasic acid (say HA) has a pH value of $5$ and concentration of $0.005M$.

We know $pH=-log\left[H^{+}\right]=5$
Hence, $\left[H^{+}\right]={10}^{-5}$

The ionisation reaction is as follows:

$HA\leftrightharpoons H^{+}+A^{-}$

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

Here, $\alpha$ is the degree of ionisation and $c$ is the concentration of an acid.
The relationship between degree of ionization and concentration is $\alpha =\frac{[H{]}^{+}}{c}$
On substituting values in the above equation, we get
$\alpha =\frac{{10}^{-5}}{0.005}=2\times {10}^{-3}$

Hence, the degree of ionisation is $2\times {10}^{-3}$.