Alkaline, pyrogallol removes \(CO _{2}\) (if present) and excess \(O _{2}\) lcaving behind only steam as the residual gas
\(2 H _{2( g )}+ O _{2( g )} \rightarrow 2 H _{2} O _{( g )} 2 mol ^{2 mol }\)
\(\left(\begin{array}{c}\text { Volume } \\ \text { that formed } \\ \text { reacted }\end{array}\right) \leftarrow: 50 cm ^{3} 25 cm ^{3} 50 cm ^{3}\)
\(\left(\begin{array}{c}\text { Excess } \\ \text { volume }\end{array}\right) \leftarrow(100-25) cm ^{3}=75 cm ^{3}\)lkaline pyrogallol removes the excess \(\left(75 cm ^{3}\right)\) oxygen, leaving \(50 cm ^{3}\) of steam as the residual gas