Calculate the intermolecular collision frequency and the mean free path in a sample of helium gas with a volume of 5.0 L at 27 celsius and 3.0 atm....

Calculate the intermolecular collision frequency and the mean free path in a sample of helium gas with a volume of 5.0 L at 27 celsius and 3.0 atm. Assume that the diameter of a helium atom is 50. pm. Also please explain how you acquire the molar mass (M)... in some examples it says 4.00 x 10^-3, but I was wondering how you come about this/get this? Please show all work. Thank you!