Question

The ionisation constant of an acid-base indicator (a weak acid) is $1.0\times {10}^{-6}$. The ionised form of the indicator is red and unionised form is blue. The pH change required to alter the colour of the indicator from 80% red is

• A. 0.8
• B. 1.2
• C. 1.4
• D. 2

Answer 1

The correct option is B 1.2

For an acid-base indicator,

$pH=p{K}_{In}+log\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}$

So, when indicator is 20% red

$\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}=\frac{20}{80}$

$p{H}_1=6+log\frac{2}{8}$

And, when indicator is 20% blue

$\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}=\frac{80}{20}=4$

$p{H}_2=6+log\frac{8}{2}$

So, $\Delta pH=2log2+2log2=1.2$

Therefore, the pH change required to alter the colour of indicator from $80\%$ red is $1.2$.