This problem relates to the Joule-Kelvin (throttling) process discussed in Section 27.3. The Joule-Kelvin coefficient is defined according to T/p.
a) For a van der Waals gas, find the inversion curve (see the dashed line in Fig. 27.3), defined to be the curve where the Joule-Kelvin coefficient is zero. Give the result in terms of the reduced variables of Problem 3, finding p' as a function of T'. You can start from Equation 27.23, and you can make use of the calculus identity: ∂V/∂(ap/aT)y = (∂p/∂(ap/aV))T. This will give you a relation for V in terms of a, b, and T, which you can then substitute back into the van der Waals equation to eliminate V. After a fair amount of algebra, you can arrive at the answer p = 9 - 12T - 3. Hint: This does take a fair bit of algebra. It is a lot easier, with less writing, if you work in the reduced dimensionless variables from the start. An intermediate result is V = -1/(1 - 3T).
b) Sketch a graph of the inversion curve, indicating quantitatively the intercepts on the T' axis as well as the point of maximum pressure.