\([O{H^ - }] = C\,.\,\alpha \) ; \(C = \frac{1}{{10}}M\), \(\alpha = 0.2\)
\([O{H^ - }] = \frac{1}{{10}} \times 0.2 = 2 \times {10^{ - 2}}\,M\)
\(pOH = - \log \,\,[O{H^ - }]\)\( = \log \,\,[2 \times {10^{ - 2}}]\); \(pOH = 1.7\)
\(pH = 14 - pOH\) \( = 14 - 1.7 = 12.30\).