An gas is sufficiently ideal when its compressibility factor #Z# is close to #1#.
The compressibility factor is #Z = (PV)/(nRT)#, and it describes the ease or difficulty in compressing the gas:
- Is the molar volume #barV = V/n# smaller than for an ideal gas? If so, #Z < 1#.
- Is the molar volume #barV = V/n# larger than for an ideal gas? If so, #Z > 1#.
When #Z < 1#, the attractive forces dominate, and when #Z > 1#, the repulsive forces dominate, when it comes to the volume of #"1 mol"# of the gas at STP (#"1 bar"#, #0^@ "C"#).
For helium, #color(blue)(Z = 1.0005)# at #"1.013 bar"# and #15^@ "C"#, so helium is close enough to ideal.
NOTE: Even if you use the Ideal Gas Law, the only thing you need to turn it into what I would call the "Real Gas Law" is the real #barV#.
The other variables, #P# (pressure) and #T# (temperature) are independent of the gas's identity.
Hence, if you know #Z# (which you can look up), you know what the real (not just ideal) #barV# is, and you've accounted for the only observable value that differentiates a real gas from an ideal gas: #barV#.
