Parameters for ideal gas are Pressure P, Volume V, number of particles N and Temperature T.


You want volume to be kept the same, and change the mass of the particles (different gas).
What else need to be the same? or changed?

If it is the same temperature T and the same number of particles, then pressure will be the same.
Because the same temperature T, means the same average kinetic energy.
So heavy particles will have less average velocity (compared to light particles).