[Timothy]

I have got a question that is asking to calculate w, g, and deltaU. The question is 1.00 kg of water is brought to its boiling point at abnospheric pressure and the heating continued until all of the water has evaporated. Calculate for the system:
(i) w (i1) q (iiD LU For water at 100 oC, the density is 958.39 kg *-' and the molar enthalpy of vaporisation is
40.6 kJ mol-l.

My idea was w = -pexDeltaV, where pexDeltaV is replaced by nRT
                     = -(1000/18.016)(8.315)(373.15)
                     = -172.2 KJ

I assumed that P was constant, as the atmosphere pressure is constant(I think), H=q(at constant pressure)
q = 40.6 kJ

Delta U = q + w
           = (40.6) + (-172.2)
           = -131.6 KJ


Are these answers plausible as  I did not use the density value?

[rabolisk]

W = PexdV is always correct for expansion work. Now the goal is to find what the change in volume of water is when it changes from liquid to gas. You only have to assume that the gas behaves ideally. Volume of liquid is given, indirectly.

Also remember that you are given molar enthalpy of vaporization and asked to find w, q, and deltaU for 1.00 kg of water.