Which R to use

[Samantha Lee 1A]

I am still a little confused on which R to use in the equation w = -nRT*ln(V2/V1) for a reversible pathway (in today's lecture). Do we use 0.0821, or 8.314. Usually pressure units is what determines which to use but I don't see pressure in this equation, so which R value would we use?

[Sophia Wendin 3L]

I think we use 8.314J/K*mol for that equation because the mol and K cancel with n and T leaving us with J.

[Ria Nawathe 1C]

You would use 8.314 because that value would give you work with units of Joules.

[Arezo Ahmadi 3J]

Adding onto what has already been said about using 8.314 J/mol*K, make sure to always write out the units of your values because even if you do not remember which R value to use, the units can really help you identify which R value will work out.

[Josh Chou 3K]

It depends on the units of the equation. For example, if you want to solve for n in the equation PV = nRT and you're given everything except R, you will most likely use 0.08206 L atm / mol K because you want the units to cancel out to give you n in mols, so you would use this constant if you're dealing with L and atm. Now, if you want to calculate the work done on a system and you're using the equation w = -nRT ln(V₂/V₁), then you would use 8.314 J / mol K because you want your answer to be in Joules, so this constant would allow you to cancel out mol and K and give you your final answer in Joules. The R value you use will depend on the units you are working with in a given problem. Also, the ratio between theses constants follows the conversion rates between L atm and J. (1 L atm = 101.325 J, which means that R = 8.314 J / mol K is 101.325 times greater than R = 0.08206 L atm / mol K)

[My-Lan Le 1L]

It depends on the problem and the units given for each problem. Usually, you can figure out which R to use by matching units and making sure they all cancel out. However, a good rule of thumb is to use 8.206*10^-2 when you are using the equation PV = nRT and 8.314 when calculating for work.

[VincentLe_3A]

In determining which value of R to use, it is important to consider the units of the other given values. You must make sure the units cancel accordingly, so you must use the R value which allows for unit cancellations.

[Mari Williams 1K]

Check with the units given in the equation to decide which R to use! It's helpful to use different Rs so you don't have to always convert the units.

[Rose_Malki_3G]

You use 8.314

[Kelly Tran 1J]

The R you would use in the equation w = -nRT*ln(V2/V1) is R = 8.314 J/(K*mol) so that the units will cancel out. When you are solving the ideal gas law equation PV=nRT, you would use R = 0.08206 (L*atm/(K*mol)).

[Lillian]

Use 8.314 in this case, but your best bet would be to look at the given units for the equation and see which one cancels out the units properly :)

[Sahaj Patel Lec3DisK]

In this equation, you would use R=8.314 J/K*m. Hope this helps!

[Victoria Dao 3G]

Like everyone said above, you just have to watch out for units!

[MariaCassol1L]

One easy way to know which R to use is by looking at the units, so R=0.08205 L atm mol^-1 K^-1 has both a pressure unit and a volume unit, therefore you need an equation that either requires some value(s) with volume and pressure or you want an answer with volume/pressure. R=8.314 J K^-1 mol^-1 has an energy unit, so if your equation has energy terms or you want an answer in Joules you should use this one. Always look at the units of your other terms and see if they cancel out to the unit of your answer.

[Lauren Mungo 1K]

you use 8.314J * K-1*mol-1 because if you look at the units, mol and K will then cancel so it leaves you with J

[Ziyan Peng 3A]

one thing that's really helped me is looking at the units! in this case 8.314 :)

[Sam Wentzel 1F 14B]

8.314 for every R outside of PV = nRT

[Charmaine Ng 2D]

You use 8.314 for this case, but in other problems, make sure to take a look to see which units would cancel out!

[Karl Yost 1L]

To choose the correct R for a calculation, use the R that will cancel out the undesired units in your calculation and leave you with your desired unit. Rule of thumb: If you are using PV=nRT, use the one with Latmmol^-1K^-1. If you are solving a question involving energy, use the one with Jmol^-1K^-1.

[Zach Richardson 2f]

Make sure you always check what units you are using first!

[alebenavides]

we use 8.314J/K*mol for those equations, hope that helps !

[Gabriel Nitro 1E]

Hi,

In order to ensure you have the proper units for entropy, you should use R = 8.314 as it has the joules (J) unit in it.

Hope this helps! :)

[Breanna Ouyang 1I]

You can usually figure out which R to use based on the units you're using to solve. If you're using PV=nRT, then it involves pressure and would need the R that includes atm, but you use 8.314 in other cases. It's easier to write out you process and all units so you know what cancels out where.

[Cora Chun 2D]

We use R=8.314 for that equation. It helps to think about the units in my opinion. Whenever it's just R by itself in the formula, it's usually 8.314, and whenever you use PV=nRT, use 8.206E-2. Hope this helps!

[Jason Knight - 1F]

After taking a look and breaking-down the problem, then you can start to get a sense of what equations/constants we want to use. Typically, we use the (R=.08206 L.atm.K^-1.mol^-1) constant for the ideal gas law equation since this compliments the units nicely. In other cases such as calculating work or entropy, we use (R=8.314 J.K^-1.mol^-1) in a normal equation requiring an R-value.

[apurva-3E]

8.314J/K*mol, but a good way is to look at the units of R values and what you are given

[Katelynn Shaheen 2C]

8.314 because your answer should be in Joules.

[Jeremy Wei 2C]

Hi for that you would use the 8.314 value. When you're not sure, I would take a look at the units when solving for the problem to see which R constant best fits your calculations.

[Charlie Russell 2L]

For future reference, look at the units you are using to see which ones you want to cancel to choose which R to use.

[Rachael Cohen 3G]

In this case you would use the 8.314 value. It helps to look at the units of the different R values and choose one that will cancel out the units of the other numbers you have, leaving you with the units you want for your answer.

[jia207]

For this example, you should use 8.314 because you would want the answer in joules

[Karina Grover 1A]

It helps to look at the units. 0.0821 has units of L atm/mol K while 8.314 has units of J/mol K. I hope this helps!

[Pratika Nagpal]

The R you need to use is dependent on the units.

[Evelyn Silva 3J]

To figure out which R to use you should always look the the units needed to be canceled. In this case you want to have your answer in Joules, so you would use 8.314

[Hannah Chang 3K]

use the one with units that align with the information given in the question

[FrancescaHawkins2H]

I struggle with this sometimes as well, and sometimes find it helps to write out the equation multiple times, using the different R values. We are supposed to use the R which best allows for units to cancel as we need to fit the problem, so although writing them out can be time consuming, it helps me to see which R will give me the correct final answer. Hope that helps!

[AndrewNguyen_2H]

A great way to know which R to use is tracking units. Use 8.314 if J is in the final answer and 0.0821 if you want L atm in the final.

[Jaden Joodi 3J]

Always use the R value that cancels outs the variables in whatever equation you are using.

[Zainab Jamali 1H]

pay attention to the units of the given values! if it’s multiple choice look at the units of the answer to give you an extra hint :)

[Ismar Zamora]

8.314!

[Kelly Ha 1K]

Use the R that corresponds with the units used in the equation you are using. The units should cancel out correctly if you used the right R value. It helps to always write out the units so it doesn't get too confusing!

[Susanna Givan 2B]

It really just depends on the units in question in your problem. This is why it's so important to make sure you're writing down the units as you go.

[LovepreetSran_3H]

You would use 8.314