can u plz tell me when is d compressibility factor (Z) in d vander waal's equation greater than 1 or less than 1??

Hi Pratyusha,

        Compressibility factor:

                        Z = PV/nRT

                (i)     Z = 1 for ideal gas

                (ii)    Z = 1 for real gases at low pressure and high temperature.

(iii)    Z > 1 show that it is difficult to copress the gas as compared to ideal gas. It is possible at high pressure. In this case repulsive force dominates. It also indicates lower value of vanderwaal’s constant ‘a’

(iv)   Z < 1 shows that the gases are easily compressible as compare dot ideal gas. It is possible at intermediate pressure. In this case attractive forces are dominant. Value of ‘a’ is high.

Note ->

(i)     In case of H, Z > 1 at 273K and Z increases with the pessure. When temperature is less than 273K, H₂ has Z < 1.

(ii)    For gases other than hydrogen, at 273K there is decrease in ‘Z’ at initial stage, with increase in pressure, value of ‘Z’ approaches a minimum with increase in pressure.