[nobOdy98]

So, I am pretty sure this is an easy problem but I seem to be over complicating it.

Calculate the entropy difference between 1mol of liquid water at 25C and 1 mol of vapor at 100C and 1bar. The Cp for liquid water can be taken constant at 75.3 J K^-1 mol^-1 and the  Hvap of water is 40.66 kJ mol^1.

I am not sure how to solve this I think I am supposed to take the water liquid:
H20(l) at 25C
H20(l) at 100C
and calculate the entropy

but I don't know what to do with the vapor. The given pressure is throwing my off as well as the Hvap.

[Dolphinsiu]

                 Delta S
H2O (l,298K) ----> H2O (g,373K)
      S1    \           /   S2
            H2O (l,373K)

Delta S = S1 + S2
          = Integrate from 298 to 373 [Cp dT/ T] + Hvap/ 373
          = 75.3 ln (373/298) + 40.66 (1000) /373
          = 125.91 J/K-mol

Actually Cp value varies with temperature. But in your question it is assumed to be constant.
Is it clear?