\([{H^ + }] = \sqrt {\frac{{{K_w} \times {K_a}}}{C}} = \sqrt {\frac{{{{10}^{ - 14}} \times {{10}^{ - 5}}}}{{{{10}^{ - 1}}}}} = \,\,\sqrt {{{10}^{ - 19}} \times {{10}^{ - 1}}} = \sqrt {{{10}^{ - 18}}} = {10^{ - 9}}\)