Question

An acid-base indicator has a $K_a=3.0\times {10}^{-5}$. The acid form of the indicator is red and the basic form is blue. Then pH is ........................ when indicator is 75% blue

• A. 4.05
• B. 7
• C. 4.35
• D. 5

Answer 1

The correct option is D 5

For an acid-base indicator

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

So when indicator is 75% red

$\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}=\frac{25}{75}=\frac{1}{3}$

$pH=5-log3+log\frac{1}{3}=4.05$

So when indicator is 75% blue

$\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}=\frac{75}{25}=3$

$pH=5-log3+log3=5$