The Value of R

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

The Value of R

Postby Kelsey Jug 1J » Sun Jan 22, 2017 7:57 pm

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

Postby rachelmackenzie3H » Sun Jan 22, 2017 8:08 pm

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
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Joined: Fri Jul 22, 2016 3:00 am
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Re: The Value of R

Postby Vincent Tse 2B » Sun Jan 22, 2017 8:11 pm

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.


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