From the graph it is clear that, the value of ' Z ' decreases with increase of pressure. We can explain as follows on the basis of van der Waals' equation. At high pressure, when ' p ' is large, V will be small and one cannot ignore ' b ' in comparison to V. However, the team
a
V2
may be considered negligible in comparison to ' p ' in van der Waals' equation. (p+
a
V2
)(V−b)=nRT p(V−b)=nRT ⇒pV−pb=nRT or p
V
n
RT=1+
pb
nRT
or Z=1+
pb
nRT
Thus, Z is greater than 1. As pressure is increased (at constant T), the factor
pb
nRT
increases. This explains why after minima in the curves, Z increase continuously with pressure. Hence, the only incorrect statement is (b).