Reviewing for the FE exam, came across questionable thermo problem.

jasc15

Im studying for the Fundamentals of Engineering exam (coming up April 12!!) and came across this Thermodynamics problem in my review book.

A gas goes through the following processes:

A to B: isothermal compression
B to C: isochoric compression
C to A: isobaric expansion

P_C = P_A = 1.4bar
V_C = V_B = 0.028m³
The net work during the C-to-A process is W_CA = 10.5kJ

What work is performed in the A-to-B process?

So the work for an isothermal process is RT_A*ln(V_B/V_A), where V_A was previously determined to be 0.103m³.

In the solution to the problem, the RT_A term is replaced with P_A*V_A, where it should be (P_A*V_A)/n from the ideal gas law.

Is this solution wrong? As I see it, this cant be solved without knowing the number of moles of gas (or mass of the gas depending on which R is used). I was unable to get the correct answer, and the solution seems wrong to me.

Mapes

The solution should have replaced nRT_A with P_AV_A. You don't need to know the amount of gas to solve this problem.

EDIT: I get -18.8 kJ for the work done in the A-to-B process.

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Mapes Recognitions: Gold Member Homework Help Science Advisor	The solution should have replaced nRT_A with P_AV_A. You don't need to know the amount of gas to solve this problem. EDIT: I get -18.8 kJ for the work done in the A-to-B process.