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
Posts: 62
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

Posts: 56
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.

Posts: 62
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
Posts: 32
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

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