Method From eqn 43.2 in the form du=v,dp, we know that the change in chemical potential of an incompressible substance when the pressure is changed by Ap is Au= V.Ap. Therefore, to answer the question, we need to know the molar volumes of the two phases of water. These values are obtained from the mass density, p, and the molar mass, M, by using Vm= M/p. We therefore use the expression Au=MApip. Method From eqn 43.2 in the form du=v,dp, we know that the change in chemical potential of an incompressible substance when the pressure is changed by Ap is Au= V.Ap. Therefore, to answer the question, we need to know the molar volumes of the two phases of water. These values are obtained from the mass density, p, and the molar mass, M, by using Vm= M/p. We therefore use the expression Au=MApip