Gas Constant R
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 116
- Joined: Wed Sep 30, 2020 9:37 pm
-
- Posts: 102
- Joined: Wed Sep 30, 2020 9:48 pm
- Been upvoted: 1 time
Re: Gas Constant R
For problems where we want a result in J, we usually use 8.314. For PV = nRT, and other types of calculations where we don't need an answer in J, we usually use a different gas constant.
-
- Posts: 23855
- Joined: Thu Aug 04, 2011 1:53 pm
- Has upvoted: 1253 times
Re: Gas Constant R
No, you want to be aware of the units.
R= 8.314 J/mol.K or R= 008206 L.atm/mol.K
Make sure you have the units of each calculation listed so you know which you need to use
R= 8.314 J/mol.K or R= 008206 L.atm/mol.K
Make sure you have the units of each calculation listed so you know which you need to use
Re: Gas Constant R
I recommend always writing out your units in your equations to make it easier to identify which R to use
-
- Posts: 103
- Joined: Wed Sep 30, 2020 9:39 pm
- Been upvoted: 1 time
Re: Gas Constant R
that's normally the case, but don't make that assumption because you don't wanna mess yourself up. always use whichever one cancels all the right units out.
-
- Posts: 103
- Joined: Wed Sep 30, 2020 9:34 pm
Re: Gas Constant R
Yes, we usually use the gas constant so we can get our final value in moles. However, we use the 8.026 times 10^-2 value whenever we try to derive any values from the ideal gas equation since those values aren't in joules.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 10:02 pm
- Been upvoted: 3 times
Re: Gas Constant R
My trick for remembering is to use 8.314 when solving for energy or joules and using 0.08206 when solving for moles, volume, temperature, or pressure.
-
- Posts: 106
- Joined: Wed Sep 30, 2020 9:39 pm
Re: Gas Constant R
usually yes because that equation doesn't account for volume (0.082 value). Write out the units for the other numbers to check for unit cancellation when you are unsure.
-
- Posts: 125
- Joined: Wed Sep 30, 2020 9:31 pm
- Been upvoted: 3 times
Re: Gas Constant R
No, you want to look at the units and make sure that the gas constant you are using applies to the corresponding units.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 10:04 pm
Re: Gas Constant R
I recommend writing out the units and then picking which gas constant works best with those units
-
- Posts: 105
- Joined: Wed Sep 30, 2020 9:39 pm
Re: Gas Constant R
General rule of thumb: use .08206 for when you are using PV=nRT, and use 8.314 for every other situation!
-
- Posts: 105
- Joined: Wed Sep 30, 2020 9:49 pm
Re: Gas Constant R
It really just depends what you want your answer to end in. 8.314 is typically used when you want your answer to end in joules.
Last edited by Allan Nguyen 2G on Mon Feb 22, 2021 1:26 am, edited 1 time in total.
-
- Posts: 151
- Joined: Wed Sep 30, 2020 9:56 pm
- Been upvoted: 2 times
Re: Gas Constant R
Hi! Not necessarily. Always make sure you write out the units you have, and observe the unit you want and make sure everything else cancels out. This has helped me make sure I don't choose the wrong one! 8.314 is usually used when you want to end in J/kJ if you convert it.
-
- Posts: 152
- Joined: Wed Sep 30, 2020 9:50 pm
- Been upvoted: 1 time
Re: Gas Constant R
The R you would use depends on the units! Use R = 8.314 J/mol.K or R = 0.08206 L.atm/mol.K depending on the units of the other terms.
Re: Gas Constant R
Depends on what you are trying to find. If joules, use 8.314, if not then you use 0.0821.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:32 pm
Re: Gas Constant R
i think the easiest way to know which value of R to use is to look at the units of the other values you are given in the question. based on that, you can determine which R value to use as your objective is to cancel things out so you are left with the correct units for your answer.
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:56 pm
Re: Gas Constant R
It really depends on what you want your end value to be in units. If you are trying to get joules, then use 8.314. Make sure the gas constant you are using works with the units you are given with and trying to find.
-
- Posts: 106
- Joined: Wed Sep 30, 2020 9:46 pm
Re: Gas Constant R
The value of R which has to be used depends on the units: 8.314 J mol-1 K-1 or 0.08206 L.atm mol-1 K-1
Also, you don't have to remember these, just look it up on the equation sheet!
Also, you don't have to remember these, just look it up on the equation sheet!
-
- Posts: 101
- Joined: Wed Sep 30, 2020 10:03 pm
Re: Gas Constant R
The R's are actually the same just calculated with differnt units, typically you use R=0.0821 for PV=nRT but if you want J then you would use R=8.314. It's all about units.
-
- Posts: 99
- Joined: Wed Sep 30, 2020 9:43 pm
Re: Gas Constant R
It depends on the units of the other variable, you want to use the R value that allows units to cancel out.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 10:09 pm
- Been upvoted: 2 times
Re: Gas Constant R
Hi!
It depends on what the units are in your calculations, so sometimes you will use R=8.314 J/Kmol and sometimes you will use R=0.08206 Latm/Kmol. Usually if you're using the PV=nRT equation for an ideal gas calculation, you'll use 0.08206 Latm/Kmol because the units match up with the other units in the equation. Hope this helps!
It depends on what the units are in your calculations, so sometimes you will use R=8.314 J/Kmol and sometimes you will use R=0.08206 Latm/Kmol. Usually if you're using the PV=nRT equation for an ideal gas calculation, you'll use 0.08206 Latm/Kmol because the units match up with the other units in the equation. Hope this helps!
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:43 pm
Re: Gas Constant R
As everyone has said it all depends on units. If the problem is asking for energy then it will most likely be the 8.314 one you use as it includes joules. If the problem is not asking for energy then you will use the other two values depending on the units for pressure
-
- Posts: 127
- Joined: Wed Sep 30, 2020 9:51 pm
Re: Gas Constant R
When working with the gas constant R, the most important thing you need to look at are the units that you have and the ones you need to find. For example, if you know the pressure, temperature, mass(in mols) and need to find the volume, you'll want to use the 8.206E-2 value.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:40 pm
Re: Gas Constant R
Hey! So initially I was confused with this concept too, but then I found that it was easiest to see what units I needed for my final answer. Then, I used the R value which allowed me to get the desired result (by cancelling out units that I needed).
-
- Posts: 106
- Joined: Mon Oct 05, 2020 12:16 am
Re: Gas Constant R
The only way I am surviving the calculations right now is referring to the units. Use the equation sheet to refer to the proper units to identify the correct steps to get the units needed.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 10:08 pm
- Been upvoted: 1 time
Re: Gas Constant R
No you have to use the R values that corresponds to the units given in the problem
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:35 pm
Re: Gas Constant R
The gas constant really depends on the units you need to cancel in order to find what you want. This can also mean changing the given value in J to kJ so that we a can minimize the amount of conversions done.
-
- Posts: 51
- Joined: Thu Dec 17, 2020 12:17 am
Re: Gas Constant R
The gas constant you use depends on the units of the given information. So far it seems that most of the problems we've been given do use the 8.314 J/ (K*mol) so I wouldn't stress about it too much. However just make sure to know which constant to use should you be given something in units that differ from this R value.
-
- Posts: 82
- Joined: Wed Sep 30, 2020 10:04 pm
Re: Gas Constant R
The gas constant you use depends on the units you have to convert to in the problem, whether they be L*atm, J, L*Torr, etc.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:39 pm
-
- Posts: 118
- Joined: Wed Sep 30, 2020 10:04 pm
- Been upvoted: 2 times
Re: Gas Constant R
It depends on what the problem is asking. You typically should look at the units to determine which gas constant R to use.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:55 pm
- Been upvoted: 1 time
Re: Gas Constant R
We use 8.314 for the nRT equations that involve work or entropy because we want the final result to be in Jules (J). However, when we just want to use the ideal gas law, we use 0.0821 because the units are atm*L/mol*K
-
- Posts: 105
- Joined: Wed Sep 30, 2020 9:31 pm
Re: Gas Constant R
We typically use 8.314 for equations involving energy change (Joules). For the ideal gas law we usually go for one of the other R constants, depending on which one best fits the units we are using.
-
- Posts: 96
- Joined: Wed Sep 30, 2020 10:09 pm
Re: Gas Constant R
Most of the time yes. But make sure to remember the units and make sure they end up canceling out.
-
- Posts: 112
- Joined: Wed Sep 30, 2020 9:39 pm
Re: Gas Constant R
The gas constant used depends on the desired units for your final answer. If it includes joules, then 8.314 is suitable.
Re: Gas Constant R
it depends on the units you used to solve the question. the two options are R= 8.314 J/mol.K or R= 008206 L.atm/mol.K
-
- Posts: 108
- Joined: Wed Sep 30, 2020 9:37 pm
Re: Gas Constant R
Taha 2D wrote:it depends on the units you used to solve the question. the two options are R= 8.314 J/mol.K or R= 008206 L.atm/mol.K
I agree! Generally, we usually use 8.314, while 0.08206 is used for PV=nRT. That's just generally though, so it's always safer to check by units
-
- Posts: 108
- Joined: Wed Sep 30, 2020 9:40 pm
Re: Gas Constant R
Not necessarily, it depends more on units. Eg: if joules are in the problem, it is probably going to be the 8.314 value for R.
-
- Posts: 102
- Joined: Wed Sep 30, 2020 9:40 pm
-
- Posts: 138
- Joined: Wed Sep 30, 2020 9:50 pm
- Been upvoted: 1 time
-
- Posts: 109
- Joined: Wed Sep 30, 2020 9:59 pm
Re: Gas Constant R
I usually look at the units given in the problems to determine which R value to use. For example, if we want our answer to be in J, use 8.314.
-
- Posts: 109
- Joined: Wed Sep 30, 2020 10:00 pm
Re: Gas Constant R
It ultimately depends on the units. 8.314 tends to be used for energy problems which deal with joules, while 0.08206 is used for problems with volume, moles, pressure, or temperature.
Return to “Gibbs Free Energy Concepts and Calculations”
Who is online
Users browsing this forum: No registered users and 7 guests