Question

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

• A. $5$
• B. $4.04$
• C. $9$
• D. none of these

Answer 1

The correct option is B $4.04$

The pH of indicator is given as follows:

$pH=pKIn+log\frac{\left[In\right]}{\left[HIn\right]}$

So, when indicator is $75\%$ red,

$pH=pKIn+log\frac{\left[In\right]}{\left[HIn\right]}=5-log3+log\frac{0.25}{0.75}=4.04$

When the indicator is $75\%$ blue,

$pH=pKIn+log\frac{\left[In\right]}{\left[HIn\right]}=5-log3+log\frac{0.75}{0.25}=5$