At constant volume, you cool a sealed, air-filled can, and the pressure reduces by half. which of the following happens? half of the gas leaks out the temp of the air reduces by half the spaces between the gas molecules shrink the number of collisions per unit area doubles

Answer is: the temperature of the air reduces by half.
The Pressure Temperature Law - the pressure of a given amount of gas held at constant volume is directly proportional to the temperature:
P₁/T₁ = P₂/T₂.
P₁ = 2P₂.
2P₂/T₁ = P₂/T₂.
T₂ = T₁P₂/2P₂.
T₂ = T₁/2.
As the pressure goes down, the temperature also goes down.