Standard temperature $-273K$t standard temperature, the steam formed condenses to liquid and so give negligible contribution to the volume of the residual gas
$2H_{2(g)}+O_{2(g)}\to 2H_2{O}_{(n)}$
$\begin{array}{lr}2mol& 1mol\\ 50c{m}^{\prime }& 25c{m}^3\end{array}$
This shows that $H_2$ is in excess by $(70-50)c{m}^3=20c{m}^3$.
That is the volume of the residual gas $\left(H_2\right)$ is $20c{m}^3$.