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