R constants

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Tatum Keichline 2B
Posts: 64
Joined: Fri Sep 28, 2018 12:26 am

R constants

Postby Tatum Keichline 2B » Tue Feb 12, 2019 11:05 pm

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?

Jim Brown 14B Lec1
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Joined: Fri Sep 28, 2018 12:28 am

Re: R constants

Postby Jim Brown 14B Lec1 » Tue Feb 12, 2019 11:10 pm

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

monikac4k
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Joined: Fri Sep 28, 2018 12:25 am

Re: R constants

Postby monikac4k » Wed Feb 13, 2019 11:24 am

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.

Shibhon_Shepard
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Joined: Wed Nov 15, 2017 3:04 am

Re: R constants

Postby Shibhon_Shepard » Tue Feb 19, 2019 9:14 pm

The best way to determine this is just to look if you can cancel out all the units.

Alysa Rallistan 2G
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Joined: Thu Jan 17, 2019 12:16 am

Re: R constants

Postby Alysa Rallistan 2G » Thu Mar 07, 2019 12:21 am

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

205389184
Posts: 100
Joined: Sat Sep 07, 2019 12:18 am

Re: R constants

Postby 205389184 » Sun Feb 02, 2020 11:26 pm

The value of R we use depends on the units of the problem you are trying to solve.

Venus_Hagan 2L
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Joined: Fri Aug 02, 2019 12:16 am

Re: R constants

Postby Venus_Hagan 2L » Sun Feb 02, 2020 11:27 pm

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

Maria Poblete 2C
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Joined: Wed Sep 18, 2019 12:15 am

Re: R constants

Postby Maria Poblete 2C » Sun Feb 02, 2020 11:40 pm

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.

Michelle Song 1I
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Joined: Thu Jul 11, 2019 12:17 am

Re: R constants

Postby Michelle Song 1I » Tue Feb 04, 2020 6:07 pm

The way you determine is by looking at the units you have.

AniP_2D
Posts: 95
Joined: Sat Aug 17, 2019 12:17 am

Re: R constants

Postby AniP_2D » Wed Feb 05, 2020 10:39 pm

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.

Jasmine Vallarta 2L
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Joined: Sat Aug 17, 2019 12:18 am

Re: R constants

Postby Jasmine Vallarta 2L » Wed Feb 05, 2020 11:12 pm

it depends on what units are given in the problem; each R value has a different set of units

Matt Sanruk 2H
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Joined: Wed Sep 18, 2019 12:21 am

Re: R constants

Postby Matt Sanruk 2H » Wed Feb 05, 2020 11:15 pm

So .08206 is used for non energy related questions?

DesireBrown1J
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Joined: Wed Sep 18, 2019 12:18 am

Re: R constants

Postby DesireBrown1J » Mon Feb 10, 2020 10:11 pm

For which problems on the equation sheet will we ever use R=8.314*10-2 L*bar*K-1*mol-1?

Donna Nguyen 2L
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Joined: Sat Aug 24, 2019 12:17 am

Re: R constants

Postby Donna Nguyen 2L » Tue Feb 11, 2020 12:46 am

It generally depends on what units are given to you in the problem, then you base which R to use off of that.

Matt Sanruk 2H
Posts: 131
Joined: Wed Sep 18, 2019 12:21 am

Re: R constants

Postby Matt Sanruk 2H » Tue Feb 11, 2020 3:18 am

So we would look at what would cancel out in order to get the answer we want for example L, atm, J

Verity Lai 2K
Posts: 100
Joined: Sat Aug 24, 2019 12:18 am

Re: R constants

Postby Verity Lai 2K » Wed Feb 12, 2020 11:11 am

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.

Ying Yan 1F
Posts: 101
Joined: Fri Aug 02, 2019 12:16 am

Re: R constants

Postby Ying Yan 1F » Sun Feb 16, 2020 12:51 pm

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!

Anish Natarajan 4G
Posts: 51
Joined: Thu Jul 25, 2019 12:16 am

Re: R constants

Postby Anish Natarajan 4G » Sun Feb 16, 2020 1:09 pm

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

pmokh14B
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Joined: Sat Aug 17, 2019 12:15 am

Re: R constants

Postby pmokh14B » Sun Feb 16, 2020 1:11 pm

it depends on the units of the other variables in the equation/calculation you are doing.

Emily Lo 1J
Posts: 50
Joined: Thu Sep 19, 2019 12:16 am

Re: R constants

Postby Emily Lo 1J » Sun Feb 16, 2020 1:15 pm

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.

Amy Kumar 1I
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Joined: Thu Jul 11, 2019 12:15 am

Re: R constants

Postby Amy Kumar 1I » Sun Feb 16, 2020 5:31 pm

Make sure your units cancel out and the unit for pressure (bar, atm, Torr) matches the unit for the pressure value in the question.

kausalya_1k
Posts: 50
Joined: Wed Nov 14, 2018 12:23 am

Re: R constants

Postby kausalya_1k » Mon Feb 17, 2020 11:55 pm

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)


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