Question

A certain acid-base indicator is red in acid solution and blue in basic solution. At $pH=5,$ 75% of the indicator is present in the solution in its blue form. Calculate the pH at which the indicator shows 90% red form?

(Given ${10}^{4.523}=3\times {10}^{-5}$)

• A. 3.56
• B. 5.47
• C. 2.5
• D. 7.4

Answer 1

The correct option is A 3.56

$pH=p{K}_1+log\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}$

$5=p{K}_1+log\frac{75}{25}$

$\Rightarrow p{K}_1=4.523$

$\Rightarrow K_1=3\times {10}^{-5}$

$pH=4.523+log\frac{10}{90}=4.523-0.954=3.56$.