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

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

PC = PA = 1.4bar
VC = VB = 0.028m3
The net work during the C-to-A process is WCA = 10.5kJ

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

So the work for an isothermal process is RTA*ln(VB/VA), where VA was previously determined to be 0.103m3.

In the solution to the problem, the RTA term is replaced with PA*VA, where it should be (PA*VA)/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.

## Answers and Replies

Mapes
Science Advisor
Homework Helper
Gold Member
The solution should have replaced nRTA with PAVA. 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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