Page 6 of TBR Gen Chem 2 explains this very well
"If the kinetic energy of the particle increases, the particle's speed increases, so it collides more frequently with the wall. Because it is moving faster, it collides with greater momentum, so impulse increases. The result on the macroscopic level is that the force per unit area exerted against the walls increase, meaning pressure is greater. The kinetic theory of gases explains macroscopic observations using principles derived from a microscopic model.
When there are many gas particles in the container, collisions between particles become more common than collisions with the wall. However, the presence of more particles in the container also results in a greater number of collisions with the walls, so the pressure of the system increases as particles are added to the system. When there are particles of different masses in a mixed gas, heavier particles move more slowly, hence they exhibit lower collision frequencies. However, because they have a greater mass and only slightly reduced speed, they collide with greater force (momentum).
As a general rule, lighter gas molecules have greater average speeds (and greater collision frequencies) than heavier ones, but less momentum (and thus less collision force). Because pressure depends on both collision frequency and collision force, gas particles of different masses exert the same pressure. On the macroscopic level, this means that pressure is the same under identical conditions for all ideal gases, independent of their molecular mass. A good example is to compare helium and nitrogen. The reason they have the same pressure at the same temperature is because they have the same kinetic energy (mv2 term). The molecule with greater mass has less speed. The average speed is inversely proportional to the square root of the mass. "
