R constants
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 64
- Joined: Fri Sep 28, 2018 12:26 am
R constants
How do we know which value of R to use because aren't there two values? I know we usually use 8.3145, but when would we use the other value?
-
- Posts: 62
- Joined: Fri Sep 28, 2018 12:28 am
Re: R constants
It depends on the constants and what needs to cancel. Both values should be provided so you just have to look at the question. It should usually be 8.314 J/k.mol
Re: R constants
Whenever using the ideal gas equation (pv=rnt) we want to use the R constant with units of L*atm*mol^-1*K^-1 because the units will cancel.
When solving for work or entropy, we usually use the one that 8.314 J/mol*K because the units cancel out and leave us with J.
In order to be absolutely sure, just take a look at the units you already have, consider the units you want for your answer, and select the R-value with units that will get you to your desired answer.
When solving for work or entropy, we usually use the one that 8.314 J/mol*K because the units cancel out and leave us with J.
In order to be absolutely sure, just take a look at the units you already have, consider the units you want for your answer, and select the R-value with units that will get you to your desired answer.
-
- Posts: 62
- Joined: Wed Nov 15, 2017 3:04 am
-
- Posts: 32
- Joined: Thu Jan 17, 2019 12:16 am
Re: R constants
One of the good things about the final is we are given the constants sheet with all the gas constants given to us, but like previously stated, the best way to approach a problem where you have to use an R constant is to look at the values and units given and use the constant with most (if not all) the units and you should be able to come to the right answer without error
-
- Posts: 104
- Joined: Fri Aug 02, 2019 12:16 am
Re: R constants
The value of R changes based on the units. All of them are given on the equation sheet, so just look to see what units the problem has given you and choose the appropriate one
-
- Posts: 102
- Joined: Wed Sep 18, 2019 12:15 am
Re: R constants
It is important to look at the units of what the problem includes. All of the R constants will be provided on the equation sheet, so when you need to use the R constant, consider what units you are using and use the constant that aligns with these units.
-
- Posts: 102
- Joined: Thu Jul 11, 2019 12:17 am
Re: R constants
A quick way of determining which R value to use is to look at the units given to you. The units you would need to cancel out should correspond with that of the R value you use.
-
- Posts: 102
- Joined: Sat Aug 17, 2019 12:18 am
Re: R constants
it depends on what units are given in the problem; each R value has a different set of units
-
- Posts: 131
- Joined: Wed Sep 18, 2019 12:21 am
-
- Posts: 98
- Joined: Wed Sep 18, 2019 12:18 am
Re: R constants
For which problems on the equation sheet will we ever use R=8.314*10-2 L*bar*K-1*mol-1?
-
- Posts: 100
- Joined: Sat Aug 24, 2019 12:17 am
Re: R constants
It generally depends on what units are given to you in the problem, then you base which R to use off of that.
-
- Posts: 131
- Joined: Wed Sep 18, 2019 12:21 am
Re: R constants
So we would look at what would cancel out in order to get the answer we want for example L, atm, J
-
- Posts: 100
- Joined: Sat Aug 24, 2019 12:18 am
Re: R constants
I always look at the units of the information given to me and find the value of R that has most of the same units.
-
- Posts: 101
- Joined: Fri Aug 02, 2019 12:16 am
Re: R constants
Look at the units of the constant and units of the problem to determine which constant to use! Usually, when doing thermodynamics we will be using 8.314. Hope that helps!
-
- Posts: 51
- Joined: Thu Jul 25, 2019 12:16 am
Re: R constants
It primarily depends on the units used; as a general rule 0.08206 works when you're using ideal gas law and 8.314 works for calculating deltaS since it does the conversion from L atm to joules for you
Re: R constants
it depends on the units of the other variables in the equation/calculation you are doing.
-
- Posts: 50
- Joined: Thu Sep 19, 2019 12:16 am
Re: R constants
You just have to make sure that you have the right units with the R value. For the most part in thermochemistry we used 8.314 but like in an instance of using PV=nRT, we would use the other R values.
-
- Posts: 114
- Joined: Thu Jul 11, 2019 12:15 am
Re: R constants
Make sure your units cancel out and the unit for pressure (bar, atm, Torr) matches the unit for the pressure value in the question.
-
- Posts: 50
- Joined: Wed Nov 14, 2018 12:23 am
Re: R constants
You would use the constant that has all the units that you need to cancel out
so if you are given a volume in L, a pressure in atm, and a temperature given in K, you would use R=.0206 L*atm/(K*mol)
so if you are given a volume in L, a pressure in atm, and a temperature given in K, you would use R=.0206 L*atm/(K*mol)
Return to “Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)”
Who is online
Users browsing this forum: No registered users and 0 guests