$K=\frac{\left[ {{H}^{+}} \right]\left[ {{F}^{-}} \right]}{\left[ HF \right]}\,\,\,\,\left[ {{H}^{+}} \right]=\left[ {{F}^{-}} \right]\Rightarrow 2\times {{10}^{-4}}=\frac{{{\left[ {{H}^{+}} \right]}^{2}}}{0/5}\Rightarrow \left[ {{H}^{+}} \right]=\sqrt{2\times {{10}^{-4}}\times 0/5}={{10}^{-2}}$  ${{\left[ HF \right]}_{(Avalyeh)}}={{\left[ HF \right]}_{(Taadoli)}}+{{\left[ HF \right]}_{(Yonesh\,Yafteh)}}=0/5+0/01=0/51\,\,\,\,\,\,,\,\,\,\,\,\,\alpha =\frac{{{10}^{-2}}}{0/51}=0/0196$