How many grams of chlorine are required to completely react
with 0.40 g of hydrogen to have HCL?

give me abc and i will get it for u.

when the pressure is increased, there will be a shift to the reactants side. decreasing the volume will increase pressure. when you decrease the volume, the gaseous species get more tightly packed of the concentrations of gaseous species increase.

le chatelier's principle says that the system wants to undo this concentration increase so it can decrease pressure.

since the reactants only have 1 mol of gas (only pcl5) and the products have 2 mol of gas (both pcl3 and cl2), it shifts to reactants to decrease concentration, thereby lowering the pressure