Question

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

• A. The constant ‘a’ is negligible and not ‘b’
• B. The constant ‘b’ is negligible and not ‘a’
• C. Both constants ‘a’ and ‘b’ are negligible
• D. Both the constants ‘a’ and ‘b’ are not negligible.

Answer 1

Answer: (A)

The constant ‘a’ is negligible and not ‘b’

Answer 2

$\left( P + \frac{n^{2}a}{V^{2}} \right)$ (V – nb) = nRT

At low pressures, ‘b’ can be ignored as the volume of the gas is very high. At high temperatures ‘a’ can be ignored as the pressure of the gas is high.

∴P (V–b) = RT

PV - Pb = RT

PV = RT + Pb

$\frac{PV}{RT} = Z = 1 + \frac{Pb}{RT}$