\(n_{H^+}=0,4.0,5=0,2\left(mol\right)\)
\(n_{OH^-}=0,6.0,5=0,3\left(mol\right)\)
\(\Rightarrow n_{OH^-dư}=0,1\left(mol\right)\)
\(\Rightarrow\left[OH^-\right]=\dfrac{0,1}{0,4+0,6}=10^{-1}M\)
\(\Rightarrow\left[H^+\right]=10^{-13}M\)
\(\Rightarrow pH=13\)