Experimental values of critical compressibility factor (Z_c) for most gases fall in a narrow range of:

Explanation:

Important points:

• Compressibility factor for ideal gas (Z) = 1 → Compressibility factor for real gas (Z) = may be less than or more than unity (Z<1 or Z>1)
• The value of Z at the critical state of a van der Waals gas is 0.375
• At very low pressures Z approaches unity, as a real gas approaches the ideal gas behaviour. 
• Experimental values of the critical compressibility factor (Zc) for most substances fall within a narrow range 0.20-0.30.

Additional Information

• The compressibility factor (Z) of any gas is a function of only two properties, usually temperature (T) and pressure(p), so that Z= f (Tr, pr) except near the critical point.
• The value of Z for any real gas may be less or more than unity, depending on pressure and temperature conditions of the gas.
• The general compressibility chart is plotted with Z versus pr for various values of Tr. This is constructed by plotting the known data of one or more gases and can be used for any gas.​