Q27 / 180 · COMEDK 2024 (Memory Based) · Chemistry
The pH at which an acid indicator with $K_\alpha=1 \times 10^{-5}$ changes colour when the indicator concentration is $1 \times 10^{-3} \mathrm{M}$ is
- A4
- Correct answerB5
- C6
- D3
Solution
An acid indicator is a weak acid. It dissociates
$\underset{\text { Colour } A}{\operatorname{HIn}} \rightleftharpoons \mathrm{H}^{+}+\underset{\text { Colour } B}{\operatorname{In}^{-}}$
$K_{\mathrm{In}}=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]}$
The indicator HIn will show a colour change at mid point, i.e. the point at which $\left[\mathrm{In}^{-}\right]=[\mathrm{HIn}]$
Hence, $K_{\mathrm{in}}=\left[\mathrm{H}^{+}\right]=10^{-5}$
or $\quad \mathrm{pH}=5$
$\underset{\text { Colour } A}{\operatorname{HIn}} \rightleftharpoons \mathrm{H}^{+}+\underset{\text { Colour } B}{\operatorname{In}^{-}}$
$K_{\mathrm{In}}=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]}$
The indicator HIn will show a colour change at mid point, i.e. the point at which $\left[\mathrm{In}^{-}\right]=[\mathrm{HIn}]$
Hence, $K_{\mathrm{in}}=\left[\mathrm{H}^{+}\right]=10^{-5}$
or $\quad \mathrm{pH}=5$
Hence, the correct answer is option: 'B'.