## 8.115

Emily Glaser 1F
Posts: 156
Joined: Thu Jul 27, 2017 3:01 am

### 8.115

Why is it not final minus initial in this problem for p1v/rt - p2v/rt?

n1 - n2 instead of n2 - n1

Jiun Yue Chung 2I
Posts: 27
Joined: Fri Sep 29, 2017 7:06 am

### Re: 8.115

You could do final minus initial, but that would give you a negative change in moles. The question is asking how much H2 was burned, which already implies that the system is losing X amount of moles. The answer is positive because you wouldn't say -X moles of H2 was burned, but rather that X moles of H2 was burned. Not sure if that answers your question or not, but I hope it helped!

Dylan Davisson 2B
Posts: 50
Joined: Thu Jul 27, 2017 3:00 am
Been upvoted: 2 times

### Re: 8.115

I believe they do initial minus final because the question asks for how many moles were burned. Moles can only exist in a positive quantity; you cannot have negative moles of any substance.
So conceptually, to see how much hydrogen is left in the tank, a positive value would result from doing initial minus final, rather than final minus initial. And since the problem asks for a positive value by way of how many moles of hydrogen were burned, it is just more convenient to calculate the full tank minus the less full tank.

Final minus initial would get you the same answer, you would just have to recognize that the moles burned were not of a negative value, but that of a positive one because you can not have negative moles of anything.

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