Question

An acid indicator with $K_a=1\times {10}^{-5}$ changes its colour when the indicator concentration is $1\times {10}^{-3}M.$ The pH at that instant is:

• A. 5
• B. 6
• C. 9
• D. 4

Answer 1

The correct option is A 5

$H{I}_n\rightleftharpoons H^{+}+I_n^{-}$
Color A Color B

$K_{In}=\frac{\left[H^{+}\right]\left[I{n}^{-}\right]}{\left[HIn\right]}$

At the midpoint of the colour range of the lndicator $\left[I{n}^{-}\right]=\left[HIn\right]$

$K_{\ln }=\left[H^{+}\right]=1\times {10}^{-5};pH=5$

i. e., at $pH=5$, the indicator will change lts colour.