Question

Bromophenol blue is an indicator with a value of $K=6.84\times {10}^{-6}$.

At what pH (nearest integer) it will work as an indicator?

Answer 1

Bromophenol blue: $HBrPh\to H^{+}+BrP{h}^{-}$

$K_a=\frac{\left[H^{+}\right]\left[BrP{h}^{-}\right]}{\left[HBrPh\right]}$

When $BrP{h}^{-}=HBrPh$ it will work as an indicator so

$K_a=\left[H^{+}\right]=6.84\times {10}^{-6}$

and $pH=-log\left[H^{+}\right]=5.16\approx 5$

Hence, the correct answer is $5$