An gas is sufficiently ideal when its compressibility factor Z is close to 1.
The compressibility factor is Z=PVnRT, and it describes the ease or difficulty in compressing the gas:
- Is the molar volume ¯¯¯V=Vn smaller than for an ideal gas? If so, Z<1.
- Is the molar volume ¯¯¯V=Vn 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, 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 ¯¯¯V.
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) ¯¯¯V is, and you've accounted for the only observable value that differentiates a real gas from an ideal gas: ¯¯¯V.
