## when do I use 3/2R??

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Hailey Boehm 2H
Posts: 71
Joined: Fri Sep 28, 2018 12:24 am

### when do I use 3/2R??

I understand that 3/2*R is used in entropy calculations for ideal monatomic gases, but I am unsure exactly as to when I should use it versus just R. When exactly do I use it?

Becky Belisle 1A
Posts: 81
Joined: Fri Sep 28, 2018 12:18 am

### Re: when do I use 3/2R??

3/2R is the C value at a constant volume. It can be used in the equation ΔS=nCln(T1/T2). In this equation, C can be substituted with Cp (constant pressure) or Cv (constant volume) depending on the problem.

Erin Kim 2G
Posts: 75
Joined: Fri Sep 28, 2018 12:26 am

### Re: when do I use 3/2R??

Change in Entropy= nCln(T2/T1). When volume is constant the value C is Cv and is equal to 3/2R. This is also the case for monatomic gases.

Irene Zhou 1E
Posts: 73
Joined: Fri Sep 28, 2018 12:18 am

### Re: when do I use 3/2R??

You would use (3/2)R to calculate the heat capacity, Cvm, of a monoatomic gas when it is at constant volume. You would use (5/2)R to calculate the heat capacity, Cpm, of a gas at constant pressure.

Michael Novelo 4G
Posts: 64
Joined: Fri Sep 28, 2018 12:28 am

### Re: when do I use 3/2R??

It's commonly used in ΔS=nCln(T1/T2) when the questions states its an ideal gas or monatomic gas, if the question ask to calculate entropy with constant volume (Cv then use 3/2R) if it is constant pressure (Cp then use 5/2R) and R is based on the units of measurement.