A piston-cylinder device initially contains 100 kg of liquid water at T = 353.15 K and P = 0.1 MPa. The device is isobarically heated, expanding against an external pressure of 0.1 MPa, until exactly 12 % of the water has vaporized, with the rest remaining in the liquid phase. How much heat was added?
A. 2. MJ
B. 35.5 MJ
C. 7.7 MJ
D. 37.5 MJ
E. Insufficient information to solve
A. Incorrect. This is the work of expansion and contraction, which is needed to solve for the requested information.
B. Incorrect. You likely made a mistake in calculating the final volume of the system or omitted the expansion work.
C. Incorrect. It appears you are interpolating to try to find the specific enthalpy of the initial state, but you are using data from two different phases. Don’t use vapor phase data in trying to find properties of a liquid.
D. Correct. The energy balance is filled in with data from the steam tables, and WEC is solved following Example 1-4.
E. Incorrect. There is sufficient information given to solve the problem. Review the definition of work of expansion and contraction if you are stuck.
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