Which R to use?

[Sristi Palimar 2E]

How do you know when to use R = 8.314 J/mol·K versus R = 0.0821 atm·L/mol·K?

[Sadhana Jeyakumar 2J]

You should be using the R value that includes the units you want to cancel out/end up with. For instance, if you want to measure pressure in bars and your equation includes volume in liters, then you would use R = 8.314 x 10^(-2) L*bar/(K*mol).

[AudreyQian1J]

Hello,

Using which R constant depends on the units you used for the rest of your equation. Those two R constants you mentioned have different units, so if your equation has units L and atm, you'd most likely use 8.206*10^-2 L*atm/K*mol instead of 8.314J/K*mol. I would recommend you write out all your units in your set up equations so you can easily determine which R constant to use that would cancel out.

I hope this helps.

[Nomi Heidari-Bateni 2K]

R is a constant. Therefore, it is one set value, but we can change its units to serve our situation. If we want to use the equation PV = nRT, then we would look at the other values in the equation, check their units, and include them in the units for R so that the units cancel when these values are multiplied/divided.

[Cory Poon 3G]

The R you use depends on the units in the problem. If you're using L*atm, use 8.206*10^-2 L*atm/K*mol and use 8.314J/K*mol when dealing with Joules.

[Shannon Lau 14B - 1H]

After going to Justin’s review session, he told us how important units are and how important it is to pick the right R in relation to the units you are trying to find. So it depends on the problem but keep in mind that the units should be able to cancel out with what you are given.

[Samantha Loc 1B]

If the problem gives units in terms of J then you would use the former, if the problem gives units in terms of atm·L then you would use the latter.

[Aaron Li 1D]

The R constant you are using will depend on the units used in the problem. In the end, you want all the units to cancel out so you're only left with the right units for the answer, so look at the units of the given values and find the R constant that matches these values' units.