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.
