Question

The pH at which an acid indicator with Ka = 1 x 10-5 changes colors when indicator concentration is 1 x 10-5 M is

• A. 4
• B. 5
• C. 6
• D. 3

Answer 1

Answer: (B)

5

Answer 2

For acid indicator Hln

Color (1) Color (2)

${K\frac{\lbrack H^{+}\rbrack\lbrack l_{n}^{-}\rbrack}{\left\lbrack H_{\ln}\lbrack\rbrack \right\rbrack}}_{\ln}$when $\lbrack l_{n}^{-}\rbrack$ = $\lbrack H_{\ln}$ indicator changes colour.

$\therefore{K +^{- 5}}_{\ln}$

$\therefore$ pH = 5