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 Waal's equation when:

• A. the constant a is negligible and not b
• B. the constant b is negligible and not a
• C. both the constants a and b are negligible
• D. both the constants a and b are not negligible

Answer 1

The correct option is A the constant a is negligible and not b

From van der Waal,s equation
$[P+\frac{a}{V^2}][V-b]=nRT$ (1)
$\because Z=\frac{PV}{nRT}$; if Z > 1 then PV > nRT
Which is possible from Eq. (1) when $a/V^2$ is negligible, i.e.,
$P(V-b)=RT$
or $PV=RT+Pb$
or $PV>RT$
So constant a is negligible.