$O{H^ - } = 0.0025 \times \frac{{50}}{{100}} \times 4$
$OH^- = 10^{-3} \times 5$ $OH^- = 5 \times 10^{-3}$
$pOH = 3 - log 5$ $ pOH = 3 - 0.69 = 2.31$
$pH + pOH = 14 $ $pH = 11.7$
$(a)$ $ 10^{-10}$ $(b)$ $\frac{{Kw}}{{{{10}^{ - 10}}}}$ $(c)$ $\frac{{Kw}}{{{{10}^{ - 8}}}}$ $(d)$ $10^{-4}$