Remember that the standard units for use in the ideal gas equation are as follows:

Pressure is in Pascals (Pa) or Nm$^{-2}$

Volume is in m$^3$

T is in Kelvins (K), which is calculated by K = 273 +$^\circ$C

If the gas is not expanded against a vacuum, think about whether it will be harder for the gas to expand or not? Consequently, think about how the energy of the gas will change, and therefore what will happen to the temperature and thus pressure.

For the final part of the problem, think about using the given radii for the two atoms. Either model each atom/molecule as a sphere or cuboid, and thus estimate its volume. Assume that the point of minimum volume is where there is no space between the atoms. Therefore, multiply the volume of a single particle by the total number of particles.