Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: miguel_91 on December 16, 2008, 01:13:43 PM

Title: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 16, 2008, 01:13:43 PM: Hi everybody, Ive tried 2 times to solve this stoichiometry problem concerning the limiting reagent, and the answer I got isn't good says the answer key. Sorry for my english.

Here is the equation: (Needs to be balanced)

Fe(OH)2 (aq) + H3PO4 (aq) → Fe3(PO4)2 (s) + H2O (l)

And the question is:

If 3.20 g of Fe(OH)2 is treated with 2.50 g of phosphoric acid, what is the limiting reagent
and what is the reactant in excess?

I always got the H3PO4 the limiting reagent but the answer key says the limiting reagent is Fe(OH)2
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: enahs on December 16, 2008, 01:18:22 PM: What is the balanced chemical equation?

How many moles of Iron(II) hydroxide do you have?
How many moles of Phosphoric acid do you have?
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 16, 2008, 04:54:26 PM: Hi enahs please help me, here is the balanced equation

3 Fe(OH)2 (aq) + 2 H3PO4 (aq) → Fe3(PO4)2 (s) + 6 H2O(l)

I shouldn't give you the moles, because that might be wrong. Do you know how to get them from the grams?

Sorry for my english.
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: enahs on December 16, 2008, 04:58:02 PM: Good, so again though,
How many moles of Iron(II) hydroxide do you have?
How many moles of Phosphoric acid do you have?

According to your balanced equation, which one do you need more of? How much more?
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 16, 2008, 05:00:54 PM: I modified my post there is all information that says the problem.
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: ARGOS++ on December 16, 2008, 05:17:13 PM: Dear miguel_91;

To continue you may apply the recipe/scheme from the Topic “Stoichiometry Problem”:
"Stoichiometry Problem (http://www.chemicalforums.com/index.php?topic=23025.0)”
Therein is also an Example Diagram: How to do a “Stoichiometry Problem”.

Good Luck!
ARGOS⁺⁺
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 16, 2008, 05:31:33 PM: Ok Argos thank u, Ill try that, but I've solved many other problems of stoichiometry well done and this doesn't work so I thought it was different and someone may help me. Ill review and say what happens.
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: Borek on December 16, 2008, 05:43:45 PM: This one is not different, however, as long as you refrain from posting numbers you get we can't tell you where you went wrong. Please show your work, step by step.
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: enahs on December 16, 2008, 05:51:34 PM: Quote
I shouldn't give you the moles, because that might be wrong. Do you know how to get them from the grams?

No, you should give the moles. I am not the one trying to get help in working the problem here!
Yes I do know how to get them from grams, do you?
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 16, 2008, 05:58:03 PM: Quote from: enahs on December 16, 2008, 05:51:34 PM
Quote
I shouldn't give you the moles, because that might be wrong. Do you know how to get them from the grams?

No, you should give the moles. I am not the one trying to get help in working the problem here!
Yes I do know how to get them from grams, do you?

Hey enahs I wasn't being though nor rude is just that Im incipient in english and I can't say what I want to say, I apologize.

Btw tomorrow Ill post my procedure =) Im busy now =(
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 18, 2008, 11:59:42 AM: [img=http://img379.imageshack.us/img379/7065/chemistrynh2.th.jpg] (http://img379.imageshack.us/my.php?image=chemistrynh2.jpg)

(then click the empty square)

as u see I got the H3PO4 the limiting reagent and the answer key says is incorrect, Ive done this 3 times and still getting the same!
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: Astrokel on December 18, 2008, 12:55:45 PM: I'm sorry, i don't understand your working. Well, in order to determine which is LR and which is in excess,

1) Calculate number of moles of the reactants
2) Divide the moles of the reactant with its stoichiometric ratio
3) The one smallest in number will be the LR and larger is the one in excess.

For example, after you calculate the moles of Fe(OH)₂, divide it with its mole ratio which is 3. You should get 0.01185. Do the same thing to the other reactant and compare which is smaller in number.
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: ARGOS++ on December 18, 2008, 01:34:35 PM: Dear miguel_91;

Sorry!, - but the program is right and your answer still wrong.

There are two causes for: (In the language of the Diagram.)
-   You did the Lines A.), B.), C₁.), and C_m.) correct, and Line C₂.) not with the required precision!
-   You did an incorrect conversion of Line C₂.) into Line D.), and with the wrong precision.

Let’s correct Line C₂.):
C_m.): 3.2 g   2.5 g
C₂.) : 0.03561 mole 0.02551 mole

I’m sure, if you do the correct conversion into Line D.) you will do also a better/more correct decision for the limiting reagent.

Conclusion: Don’t round intermediate results!, - round only the final result to the desired precision!

I hope to have been of help to you.
Good Luck!
ARGOS⁺⁺
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: miguel_91 on December 18, 2008, 07:04:28 PM: Hi Argos my procedure is different because I didn't wanted to use the rule before stated that u need to divide the moles of the reactants with the stoichiometric ratio. I believed that it was difficult to remember so I used another way that I see now that is wrong thank u all. Sorry for my english
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: ARGOS++ on December 18, 2008, 07:21:48 PM: Dear miguel_91;

That’s exactly what you have to do to convert the Line C₂.) into the Line D.):
Divide the found moles by the factors you found from the balanced Rx-equation in Line B.),
and you will already be at the goal.

So the procedure is not different, only much more consequent!
Good Luck!
ARGOS⁺⁺
Title: Re: Which is the limiting reagent? Stoichiometry
Post by: ARGOS++ on December 18, 2008, 08:13:54 PM: Dear miguel_91;

The Diagram/Scheme would results in:

B.) : 3Fe(OH)₂ + 2H₃PO₄ ---> 1Fe₃(PO₄)₂ + 6H₂O
C₁.) : 89.85 g/mole 98.0 g/mole
C_m.): 3.2 g 2.5 g
C₂.) : 0.03561 mole 0.02551 mole
D.) : 0.01187 0.01275

And now which is the limiting reagent (the lower multiplier on D.))?

You see that with the Diagram on: "Stoichiometry Problem (http://www.chemicalforums.com/index.php?topic=23025.0)” it can be such easy!
Good Luck!
ARGOS⁺⁺