# Homework Help: Derive Entropy Change for Ideal Monatomic Gas

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1. Nov 14, 2014

### SalfordPhysics

A Monatomic gas passes from state 1 (pressure p1, volume V1) to state 2 (p2, V2).
Derive an expression for the change in entropy of a monatomic ideal gas.

The required final equation is: ΔS = Cv ln(T2/T1) + nRln(V2/V1)

In my attempt, I am retrieving ΔS = Cv ln(T2/T1) + Rln(V2/V1)
i.e.; the unit of moles not present in the "(V2/V1)" term.
Is the given form to obtain incorrect or am I making a very silly mistake to do with the prior assumptions?

2. Nov 14, 2014

### Orodruin

Staff Emeritus
It is very difficult to help you if you do not state how you are arriving at your expression.

For the future, also leave the headings of the homework template in your post.

3. Nov 14, 2014

### Staff: Mentor

What are the units of Cv, R, and ΔS?

What is the exact statement of the problem?

Chet

4. Nov 15, 2014

### SalfordPhysics

Its not a problem now I figured things out, the problem was my solved form wasn't exactly as required, but like I thought I was making a mistake with a moles = 1 assumption in another derivation that i was using a a "subroutine".