City-Data Forum general stechiometry question (websites, problem)
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05-23-2009, 06:57 PM
 Location: USA 3,966 posts, read 9,151,846 times Reputation: 2197

The book i've been reading claims I need to multiply the mole ratio by the moles of reactant to get my answer to the next step. I tried to use the ratio in a limited reactants environment and receive the wrong answer everytime. When i don't include the ratio, i receive the right answer.

Am i doing something wrong or is it different for limited reactants?

Last edited by shiphead; 05-23-2009 at 07:08 PM..

05-24-2009, 10:03 AM
 Location: Westwood, MA 3,447 posts, read 4,350,324 times Reputation: 4431
I'm no chemist, but I might be able to help if you are more specific about your particular problem. Are you trying to figure out the equilibrium ratio of reactants and products? Is the product of this reaction the reactant for some other reaction?

I can guess at what you mean (although I'm unlikely to be right). Is your reaction of the form:

AX + BY -> CZ

where A, B, C are numbers X and Y are reactants and Z is the product? If that is the case, the number of moles of Z (call this N) will be given in terms of the number of moles of X and Y (L and M, respectively) by N = C*min(L/A, M/B). The answer will be different if the reaction is reversible or there are other things the reactant could be doing, but if that's the case you'll have to ask someone else!

05-24-2009, 10:35 AM
 Location: USA 3,966 posts, read 9,151,846 times Reputation: 2197
The math problem is 2 h2 + 02 -> 2 h20
The problem i am trying to figure out how much H20 can be made out of 15 hydrogen and 10 oxygen. All the websites i see talking about stechiometry do not use the ratio, but they are not using it in a limited environment.

Crap.. I think i just realized my mistake... update thread later.

Last edited by shiphead; 05-24-2009 at 10:50 AM..

05-24-2009, 09:06 PM
 Location: Texas 5,070 posts, read 8,765,860 times Reputation: 1627
7?

05-25-2009, 05:51 AM
 Location: Westwood, MA 3,447 posts, read 4,350,324 times Reputation: 4431
15/2 if moles, 7 if molecules.

05-25-2009, 10:48 PM
 Location: United States of America 10 posts, read 15,019 times Reputation: 16
The ratio between the hydrogen, oxygen, and water molecules is: 2:1:2. For every mole of water, there must be at least 1 mole of hydrogen and 1/2 mole of oxygen. With 15 mol H2 and 10 mol O2, the ratio is not 2:1, thus either H2 or O2 is a limiting reagent- aka one of the two reactants will be used up before the other. The key is calculate which one is the limiting reagent, as if it were H2, then due our 2:2 or 1:1 ratio with water, there would be 15 mol of H2O- though if it were O2, with a ratio of 1:2 with water, there would be 20 mol of H2O.

So to calculate this, pick one of two reactants as if were the limiting reagent- here I'll choose H2:
15 mol H2 * (1 mol O2)/(2 mol H2) = 7.5 mol of O2- however, there are 10 mol of O2. Therefore, O2 will be in excess (by 10-7.5 = 2.5 moles) and H2 is the limiting reagent. 15 mol of H2 will be used up and the reaction will stop, in which again from the ratio, 15 mol of H2O will form.

05-30-2009, 11:38 AM
 Location: USA 3,966 posts, read 9,151,846 times Reputation: 2197
It's asking how much h20 can be formed from 15 hydrogen or 10 oxygen. And my question is do you use mole ratio when using limited factors or more than that?
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