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 .

Options

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

A

Explanation

`(P+(n^(2)a)/(V^(2)))(V-nb)=nRT`
At low pressure, `b` can be ignored as the volume of the gas is very high. At high temperature `a` can be ignored as the pressure of the gas is high.
`:. P(V-b)=RT`
`PV-Pb=RT`
`PV=RT+Pb`
`(PV)/(RT)=Z=1+(Pb)/(RT)`