$\begin{array}{ll}50\, ml & 1\, M \\ 1\, M & V =?\end{array}$

$\Rightarrow \frac{ n _{ NaoH }}{ n _{ H _{3} PO _{3}}}=\frac{2}{1}$

$\Rightarrow \frac{1 \times V }{50 \times 1}=\frac{2}{1} \Rightarrow{ V _{ NaOH }=100\, ml }$

$H _{3} PO _{2}+2 NaOH \rightarrow NaH _{2} PO _{3}+ H _{2} O$

$\begin{array}{lc}100\, ml & 1\, M \\ 2\, M & V =?\end{array}$

$\Rightarrow \frac{ n _{ NaoH }}{ n _{ H _{3} PO _{3}}}=\frac{1}{1} \Rightarrow \frac{1 \times V }{2 \times 100}=\frac{1}{1} \Rightarrow V _{ NaOH }=200\, ml$