## R constants

Tatum Keichline 2B
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?

Jim Brown 14B Lec1
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

monikac4k
Posts: 56
Joined: Fri Sep 28, 2018 12:25 am

### 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.

Shibhon_Shepard
Posts: 62
Joined: Wed Nov 15, 2017 3:04 am

### Re: R constants

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

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

Return to “Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)”

### Who is online

Users browsing this forum: No registered users and 1 guest