If $50 \mathrm{~mL}$ of $0.5 \mathrm{M}$ oxalic acid is required to neutralise $25 \mathrm{~mL}$ of $\mathrm{NaOH}$ solution, the amount of $\mathrm{NaOH}$ in $50 \mathrm{~mL}$ of given $\mathrm{NaOH}$ solution is ______ g.