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EastWindBreaks

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## Homework Statement

An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 150 kPa. An electric resistance heater inside the cylinder is now turned on, and 2200 kJ of energy is transferred to the steam. Determine the entropy change of the water during this process

## Homework Equations

Steam tables

h_f1= specific enthalpy of saturated water in state 1

h2= specific enthalpy in state 2?

h_g2= specific enthalpy of saturated vapor in state 2

h_f2=specific enthalpy of saturated liquid in state 2 =h_f1

## The Attempt at a Solution

My first approach:

Step 1: since it has moving boundary work, Q_in=ΔU+W=ΔH=m(h2-h_f1), using steam tables to find the specific volume and specific enthalpy of saturated water at 150 kPa use them to find h2

Step 2: with P=150 kPa, and h2 is known, I need to know which phase is the water in, so I go to saturated water table and I see h_f< h2 <h_g, so it is saturated-liquid mixture, I get to use specific enthalpy of saturated water and vapor from saturated water table. find quality X from h2= h_f2+X(h_g2- h_f2)

( correct me if I am wrong please)

Step 3: with quality X known, I can find s2 from s2= s_f+X(s_fg), and those properties are from saturated water table as well. then finally ΔS= m(s2-s1)=5.72 KJ/K

when I try to check my solution, I saw this approach from chegg:

this approach looks more straightforward, but I am confused on how its using latent heat of vaporization to check if water change phase or not, since latent heat of vaporization requires constant temperature process but the temperature information is not given, and it concludes the temperature is constant from no phase change, though, temperature can change without phase change right?

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