Units Confusion  [ENDORSED]

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Jasmine Wu 1L
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Joined: Thu Jul 27, 2017 3:00 am
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Units Confusion

Postby Jasmine Wu 1L » Sat Feb 10, 2018 11:36 am

Hello! Hopefully I'm not alone with this, but I feel that there are so many equations in Chapter 8, 9, and 11! A lot of them look quite similar, too.

How would you know when to use what the book describes as "molar" convention (i.e. which formula to use). Sometimes I see that the units of the final answers are different, but the equation used are the same (or at least look pretty similar). To clarify, I see that one answer's units could be in kJ/mol and the other to be just kJ, but the equation used look alike.

Remi Lathrop 1G
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

Re: Units Confusion

Postby Remi Lathrop 1G » Sat Feb 10, 2018 12:20 pm

When you're confused on units, usually the best thing to do is to track your units throughout the problem. This means writing out all the units for every number you put into your equation and then determining which units cancel out in the final answer. It also might be helpful to review which units typically go with which terms, for example ∆S is often in J/K or J/mol K depending on the problem, whereas ∆G and ∆H are often in kJ/mol.

The more problems you do the easier it should get, hope this helps!

MSkye Goldwater 2K
Posts: 19
Joined: Fri Sep 29, 2017 7:05 am

Re: Units Confusion

Postby MSkye Goldwater 2K » Sat Feb 10, 2018 1:35 pm

I'm having the same problem. For example with homework problem 9.19 the equation delta-S = nCln(T2/T1) is used. However, in this case they just ignored the n so the resulting answer ended up in J/(k x mol). I thought that when they don't give the number of moles and you are using this equation, you should assume 1 mole so the unit would cancel out and you would end up with J/k (which I have seen as an answer in certain problems). I'm confused regarding when you should assume 1 mole and cancel out that unit, and when you shouldn't assume 1 mole and leave it in the final answer. In both cases the same equation seems to be used.

Wenxin Fan 1J
Posts: 53
Joined: Thu Jul 13, 2017 3:00 am

Re: Units Confusion  [ENDORSED]

Postby Wenxin Fan 1J » Sat Feb 10, 2018 5:55 pm

Use dimensional analysis to track your units and see which ones cancel out in your problem.

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