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

Posted:

**Tue Feb 12, 2019 9:04 pm**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?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=129&t=42099

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Posted: **Tue Feb 12, 2019 9:04 pm**

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?

Posted: **Tue Feb 12, 2019 9:08 pm**

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.

Posted: **Tue Feb 12, 2019 9:52 pm**

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.

Posted: **Tue Feb 12, 2019 9:56 pm**

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.

Posted: **Tue Feb 12, 2019 9:59 pm**

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.