For a fixed mass of gas of constant volume, the pressure is directly proportional to the absolute temperature (in kelvin) orp/T = constant If a graph of pressure against absolute temperature is plotted then a straight line through the origin is obtained.Molecules of a gas move at random, with different speeds. The molecules periodically collide with the walls of their container, exerting a force on the container. The total force per unit area is the pressure. When a gas is heated, the molecules gain kinetic energy and therefore more internal energy. They collide with the walls of the container more often and with a greater force.