## Limiting Reagent and Stoichiometry CR video

cassie
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Joined: Sat Nov 18, 2017 1:04 pm

### Limiting Reagent and Stoichiometry CR video

Slide 7 quiz: stoichiometry question 2/4
True or False: Given this reaction, 80g of oxygen is enough to produce 4 moles of product.

how do we know that only 2.5 moles of O2 is the form of oxygen used in this reaction? also the explanation says 5 moles of O2 would be necessary to produce 4 moles of HNO3 is it supposed to be 7 O2 since there is a total of 7 O2 molecules on the reactant side?
NS_Tutor_Andrew
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

### Re: Limiting Reagent and Stoichiometry CR video

Hi Cassie,

Thanks for the question! In general, when you hear references to oxygen as a product or reactant in a chemical reaction, it's pretty safe to assume that it's O2, because that's the form in which oxygen occurs in nature. This is also true for hydrogen, nitrogen, fluorine, chlorine, bromine, and iodine. Additionally, the reaction given specifies that O2 is used.

We need to distinguish the reactant oxygen (O2) from other molecules that might contain an oxygen atom. When we balance the equation, we do need to account for all atoms of oxygen, regardless of what molecule they're in -- so we will see in this equation 12 oxygen atoms on each side -- but when you read something like "80 g of oxygen is enough to produce 4 moles of product", you don't have to worry about the fact that water contains an oxygen atom too.

Hope this clarifies things, and keep up the great questions!
Andrew D.
Content Manager, Next Step Test Prep.
jzwerling
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Joined: Mon Apr 02, 2018 5:29 pm

### Re: Limiting Reagent and Stoichiometry CR video

Hi all,
I have a question that is also related to the CR Quiz Outline #7: stoichiometry question 2/4-

I checked to see that the equation was balanced, which illustrated 12 atoms of Oxygen on both sides. I chose false because it would be 16 * 12=192 grams, which shows us that 80g is not enough to produce 4 moles of product.

I chose the correct answer, but when I read the explanation, I don't understand it and I am unsure if my reasoning for choosing false is correct as well.
Can you explain if I am understanding the question and approaching it correctly? Also, can you explain the explanation that is given in the quiz?
Also, how do you know when to take into account all the moles of Oxygen versus just the moles of 02?

Thank you so much,
Jordana (jordana.zwerling@gmail.com)
NS_Tutor_Bryan
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Location: Tucson, AZ
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### Re: Limiting Reagent and Stoichiometry CR video

jzwerling wrote:Also, how do you know when to take into account all the moles of Oxygen versus just the moles of 02?

This question is the key takeaway from that particular problem. Remember that when you're reviewing any question/passage/video/full length the thing you always want to be focused on is getting a good Lesson Learned from the problem.

In this case, a great Lesson Learned from that practice problem is this: when they just say "oxygen" in chemistry problems, assume they're talking about molecular oxygen (and make that same assumption with any gas that is diatomic in its standard state: H2, N2, O2, F2, I2, Cl2, Br2).

So when the problem says "80 g of oxygen" then read that as "80 g of O2"

And then just do what you'd always do in chemistry - start by converting to moles:

80 g of O2 x (1 mol of O2 / 32 g) = 2.5 mol of O2