Question

2- A certain amount of H2O in a closed rigid cylinder is cooled from T1 = 800 ° F and P1 = 100 lbf / in2 to P2 = 20 lbf / in2. Determine the specific entropy change in Btu / (lb ° R).

Answer 1

The specific heat of water at higher temperature is 4.767 X 10-4 Btu/(lb.°R).

The change in entropy in isochoric process ( in which volume remains unchanged) is given by

..............(1)

All symbols has their ususal meaning.

Here we need the value of n (total number of moles of H2O present in the container) too.

Let the to m mass of H2O is present in the container. The molecular weight (M) of the water is 18.02 gram/mol.

Therefore, the total number of moles of water in the container is

Now, we can modify equation (1) as

The specific entropy is defined as

Therefore, the specific entropy change in the given problem is