Problem solving state change exercise

[M Wierckx]

Hi everybody,

I'm having a problem with the following question:

A piston-cylinder device originally contains 5 kg of saturated-liquid water at 100 kPa. Determine the heat addition required to bring the fluid to a saturated-vapor state.

Using the first law.. i was thinking the following:

1Q2 = 1W2 + (U2 - U1)

U2 and U1 could be found in the table of liquid water with a pressure of 100 kPa, with U2 being Saturated vapor (Ug) and U1 saturated liquid (Uf).

The W (work) part I was thinking about the P x (V2-V1) again using the table, V2 being sat. vapor (vg) and V1 being sat. liquid (vf)


Doing this I'm not finding the right answer.. can anybody help me solving this and telling me which step I'm taking wrong?

 

[RTW69]

If the piston isn't rising there isn't any work being done so w=0 and q= ΔU. See if that works. I would pick Internal Energy values off of a Saturated Water: Pressure Table.

 

[M Wierckx]

I think the piston can be moved, or else they would have made it a rigid tank.. I tried leaving the W out and making Q equal to U2-U1, but this is not correct.. The answer has to be eleven dot something (11,...kJ/kg).. but thanks for the reply!

 

[RTW69]

For a constant batch system Q=ΔU+W where W= P(V_g-V_f)