The behaviour of a real gas is usually depicted by plotting compressibility factor `Z` versus `P` at a constant temperature At high temperature and high pressure `Z` is usually more than one This fact can be explained by van der Waals' equation when .

The gases which strictly follow the general equation (PV = nRT) are called ideal or perfect gases. Actually, there is no gas which is perfect or ideal. A real gas is one which actually exists, whether it obeys gas laws strictly or not. Under ordinary conditions, only those gases nearly behave as ideal or perfect which have very low boiling points such as nitrogen, hydrogen ect. The most easily liquefiable and highly soluble gases such as ammonia, carbon dioxide, sulphur dioxide show large deviation A very convenient method of studying deviation of real gases from ideal behaviour is through a compressibility factor (Z) Z = (PV)/(nRT) (i) Z = 1 , for ideal gases. (ii) Z != 1 , for real gases. The behaviour of a real gas is usually depiected by plotting compressibility factor Z versus pressure P at a constant temperature. At high temperature and pressure, Z is usually more than one. This fact can be explained by van der Waal's equation when :

At high temperature and low pressure van der Waal's equation becomes

At high temperature and low pressure van der Waal's equation becomes

At high temperature and low pressure the van der Waals equation is reduced to .

Which of the following represents a plot of compressibility factor (Z) versus P at room temperature for helium?