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R constants

Posted: Tue Feb 12, 2019 11:05 pm
by Tatum Keichline 2B
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?

Re: R constants

Posted: Tue Feb 12, 2019 11:10 pm
by Jim Brown 14B Lec1
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

Posted: Wed Feb 13, 2019 11:24 am
by monikac4k
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.

Re: R constants

Posted: Tue Feb 19, 2019 9:14 pm
by Shibhon_Shepard
The best way to determine this is just to look if you can cancel out all the units.

Re: R constants

Posted: Thu Mar 07, 2019 12:21 am
by Alysa Rallistan 2G
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

Re: R constants

Posted: Sun Feb 02, 2020 11:26 pm
by 205389184
The value of R we use depends on the units of the problem you are trying to solve.

Re: R constants

Posted: Sun Feb 02, 2020 11:27 pm
by Venus_Hagan 2L
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

Re: R constants

Posted: Sun Feb 02, 2020 11:40 pm
by Maria Poblete 2C
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.

Re: R constants

Posted: Tue Feb 04, 2020 6:07 pm
by Michelle Song 1I
The way you determine is by looking at the units you have.

Re: R constants

Posted: Wed Feb 05, 2020 10:39 pm
by AniP_2D
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.

Re: R constants

Posted: Wed Feb 05, 2020 11:12 pm
by Jasmine Vallarta 2L
it depends on what units are given in the problem; each R value has a different set of units

Re: R constants

Posted: Wed Feb 05, 2020 11:15 pm
by Matt Sanruk 2H
So .08206 is used for non energy related questions?

Re: R constants

Posted: Mon Feb 10, 2020 10:11 pm
by DesireBrown1J
For which problems on the equation sheet will we ever use R=8.314*10-2 L*bar*K-1*mol-1?

Re: R constants

Posted: Tue Feb 11, 2020 12:46 am
by Donna Nguyen 2L
It generally depends on what units are given to you in the problem, then you base which R to use off of that.

Re: R constants

Posted: Tue Feb 11, 2020 3:18 am
by Matt Sanruk 2H
So we would look at what would cancel out in order to get the answer we want for example L, atm, J

Re: R constants

Posted: Wed Feb 12, 2020 11:11 am
by Verity Lai 2K
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.

Re: R constants

Posted: Sun Feb 16, 2020 12:51 pm
by Ying Yan 1F
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!

Re: R constants

Posted: Sun Feb 16, 2020 1:09 pm
by Anish Natarajan 4G
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

Posted: Sun Feb 16, 2020 1:11 pm
by pmokh14B
it depends on the units of the other variables in the equation/calculation you are doing.

Re: R constants

Posted: Sun Feb 16, 2020 1:15 pm
by Emily Lo 1J
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.

Re: R constants

Posted: Sun Feb 16, 2020 5:31 pm
by Amy Kumar 1I
Make sure your units cancel out and the unit for pressure (bar, atm, Torr) matches the unit for the pressure value in the question.

Re: R constants

Posted: Mon Feb 17, 2020 11:55 pm
by kausalya_1k
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)