Two different gases, at STP, have the same number of particles. What can you say about the respective volumes of the two gases?

Well, all we can say is that their volumes are dependent on their intermolecular forces of attraction... which lead to different densities.

If both gases are at STP, they share the same temperature and system pressure, and if the same number of particles of each gas are in the closed container, then the only variable left is their volumes.

The molar volume #V/n -= barV# is dependent on the intermolecular forces present in the gas. More attractive forces = smaller volume for the same number of particles.

EXAMPLE

At STP, #"NH"_3#, which has a density of #"0.769 g/L"# (a molar volume of #"22.147 L/mol NH"_3#) would have a smaller volume than #"CH"_4#, which has a density of about #"0.716 g/L"# (a molar volume of #"22.406 L/mol CH"_4#).

This is primarily because #"NH"_3# has hydrogen-bonding, which pulls the molecules in, decreasing the effective volume of #"NH"_3# compared to #"CH"_4# and making #"NH"_3# samples more dense at the same #T# and #P#.

[They have similar molar masses, which minimizes that as a factor.]

If for some completely unexpected reason, the gases were BOTH ideal, then they would have the same volume, but that's not stated...