When the system is kept at constant pressure and temperature the volume is directly proportional to the number of moles of ideal gas introduced into the container. This can clearly be seen on the graph as the number of moles of gas are changed. An increase in the number of particles will increase the number of collisions with the walls of the container. This increases the internal pressure. Since the external pressure is constant the volume of the container must increase so the internal pressure equals the external pressure. Since the temperature is kept constant, the molecules are moving with the same average speed. Compare the system before and after a changing the numeber of moles of helium and/or neon.

To further explore the relationship between pressure, volume, moles of gas and temperature look at the Gas Law MoLE. Be sure to print out a copy of the Gas Law Activity, or look at it in your Chemistry Activity Manual.