Tips for what gas constant to use

[EmilyC_3D]

I sometimes mix up what gas constant I should use. I was just wondering if anyone had any tips to easily keep track of/identify which gas constant to use so that you don't get them mixed up while trying to solve a problem!

Thank you!

[lauren_tran_1L]

You can look at the units for each different R and match it to your problem!

[Ryan Hoang 1D]

I 100% agree with Lauren, looking at the units helps a lot! But, I also remember to use the 8.31 value when dealing with work, but when trying to find out moles/volume/etc in order to get to actually solve that work problem, I'd normally use 0.08206. However, Lauren's tip of looking at units is probably the most helpful!

[Chudi Onyedika 3A]

You should just look at the units that are provided and see which R value contains them. Everything should cancel out to get your desired units.

[Bella Wachter 1A]

I think it's easiest to pay attention to the units of the constants.

Whenever you use the ideal gas law, PV = nRT, you use 0.08206 Latm/Kmol, because in this equation, P is in atm, V is in L, n is in moles, and T is in Kelvin. When using the ideal gas law, we are not calculating energy - so we don't want to use the 8.314 value because it involves Joules (also it doesn't involve volume either - its units are J/Kmol)

However, whenever we are calculating anything related to energy, such as work (units in Joules) or entropy (units in Joules / Kelvin), we want to use the 8.314 J/Kmol constant. That means that for the -nRTln(V2/V1) equation, despite the relation to the ideal gas law (the -nRT), we still use 8.314, because we want an answer in energy units. Additionally, we don't have to worry about the lack of a L unit in the constant because the liters cancel out in the natural log expression.

[Kiyoka Kim 3C]

I agree with the above comments! If you look at the units in your equation you can get a sense of which R constant to use. I also usually write out all the units when calculating entropy and etc to keep track of the units.

[Jamie2002]

Look at the units in the problem you are working on. If Joules are involved, use 8.314 J/(K x mol); if atm is involved use 0.08206 (L x atm)/(mol x K)

[Sara Sandri 2B]

I used to have the same exact problem of not knowing what gas constant to use but something I really recommend is writing down the units of everything in the problem and when you're doing calculations so you can cross things out and see what you are left with. That's helped me a lot in figuring out what exactly I'm doing.

[Anastasia Yulo 1C]

keeping track of the given units of the problem helps to determine with gas constant to use!

[Diana Aguilar 3H]

Like many others have said, you should try to keep track of/look at the units you are given in the equation to help you determine what gas constant to use!

[kristinalaudis3e]

look at the units given and find the equations that fit that!