## The Value of R

Kelsey Jug 1J
Posts: 38
Joined: Wed Sep 21, 2016 2:59 pm

### The Value of R

what is a good rule of thumb for knowing which value of R we should use? (technically same value but just in different units)

rachelmackenzie3H
Posts: 10
Joined: Fri Jul 22, 2016 3:00 am

### Re: The Value of R

Basically just look at the units that you are are trying to solve for. For example, since this unit we are focusing a lot on energy, you will be using the R=8.314J/K*mol for a majority of the calculations because this value includes Joules. However, the other values of R are more helpful in using the ideal gas law because they include units of pressure (the .08206 value is the most common because it is in atm).

Vincent Tse 2B
Posts: 30
Joined: Fri Jul 22, 2016 3:00 am
Been upvoted: 3 times

### Re: The Value of R

It usually depends on the context of the problem and the kind of system you're dealing with.

For example, if you're calculating work for a reversible gas expansion, you'll need to know moles, temperature (most likely in kelvins), and the final/initial volumes. Disregarding the volume units--they are not of concern in the process of achieving desired units for this problem--you now need to consider the R constant.

You would want to choose the R constant that will help you cancel out the units of moles and kelvins to get the unit you want: Joules, which is the appropriate measure of unit for calculating work. Therefore, for such a problem, using R = 8.314 J.K^-1.mol^-1 is best.

tl;dr -- Just be cognizant of the units you are given, the context of the problem, and the desired unit you want to have in the end.