$M+x=3(M-x)\Rightarrow M+x=3M-3x\Rightarrow -2M=-4x\Rightarrow x=\frac{M}{2}=\frac{0/1}{2}=\frac{1}{20}mol.{{L}^{-1}}$ $\left[ {{H}^{+}} \right]=\frac{1}{20}mol.{{L}^{-1}}\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-pH}}\Rightarrow \frac{1}{20}={{10}^{-pH}}$ ${{10}^{-1}}\times {{2}^{-1}}={{10}^{-pH}}\Rightarrow {{10}^{-1}}\times {{10}^{-0/3}}={{10}^{-pH}}\Rightarrow pH=1/3$