\(n_{H^+}=0.5\cdot\left(0.1+0.3\cdot2\right)=0.35\left(mol\right)\)
\(V_{dd}=a\left(l\right)\)
\(n_{OH^-}=a\cdot\left(0.3+0.2\cdot2\right)=0.7a\left(mol\right)\)
\(H^++OH^-\rightarrow H_2O\)
\(\Rightarrow0.7a=0.35\)
\(\Rightarrow a=0.5\left(l\right)=500\left(ml\right)\)