(2) 3.
How would you calculate the change in entropy when the pressure of a perfect gas is
changed isothermally from P; to Pr

How would you calculate the change in entropy when the pressure of a perfect gas is changed isothermally from P_i to P_f

ΔS = nRIn(P_i/P_f)

Explanation:

Formula for entropy change is;

ΔS = nC_p•In(T_f/T_i) + nRIn(P_i/P_f)

Since the pressure of the perfect gas is changed isothermally, it means that temperature remains the same and as such T_f = T_i

Thus, we now have;

ΔS = nC_p•In(1) + nRIn(P_i/P_f)

In(1) = 0

Thus;

ΔS = nRIn(P_i/P_f)