Question

2so2 +o2 =2so3 what would be the volume of gases after reaction? If initially 10 lit each so2 and o2 were taken and the reaction is 50 % completed!

Answer 1

Considering constant temperature and pressure, we can perform calculations on volumes of gases just as we do for moles.
$\begin{tabular}{c c c c c c} & 2SO_2 & + & O_2 & \rightleftharpoons & 2SO_3 \\ \text{Initial} & 10L && 10L && 0L \\ \text{Final} & 10L-2V && 10L-V && 2V \\ \end{tabular}$
Now, $100\%$ completion of the reaction means that concentration of $SO_2$ becomes zero. (Since, $SO_2$ is the limiting reagent.)
So $50\%$ completion of the reaction means that equilibrium volume of $SO_2$,
$V_{SO_2}=5L\\ \Rightarrow 10L-2V=5L\\ \Rightarrow V=2.5L\\ \Rightarrow V_{O_2}=10L-2.5L=7.5L\\ \text{and } V_{SO_3}=2V=5L$