\(HA⇌{H^ + } + {A^ - }\)
\([{H^ + }] = \sqrt {\frac{{{K_a}}}{C}} \) \( = \sqrt {\frac{{{{10}^{ - 9}}}}{{0.1}}} \) ; \([{H^ + }] = {10^{ - 4}}\)
\(\therefore \,pH = 5\)
\(pH+pOH = 14\)
\(pOH = 14 - pH\)\( = 14 - 5\) ; \(pOH = 9\)