Answer
(a) All concentrations will stay the same.
(b) $H_2$ and $O_2$ concentrations decrease, and $H_2O$ concentration will remain constant, since it is a liquid.
Work Step by Step
Decreasing the volume of the container shifts the equilibrium toward the side with less moles of gas.
(a) Reactants: 2 moles of gas; Products: 2 moles of gas.
The volume does not affect equilibrium.
All concentrations will stay the same.
(b) Reactants: 3 moles of gas; Products: 0 moles of gas.
Products are favored.
$H_2$ and $O_2$ concentrations decrease, and $H_2O$ concentration will remain constant, since it is a liquid.
