Solving Stoichiometry Problem on Oxalate Mass Percent

[babbagee]

I am taking chemistry 1A again just to refresh my skills. The teacher was telling us how to figure out the mass composition of oxalate in some crystals that we had just synthesized. The equation is written below

2MnO₄^- + 5C₂O₄^2- + 16H⁺ ----> 2Mn²⁺ + 10CO₂ + 8H₂O

The question is to find the Mass percent of oxalate in the green crystals. The molarity of MnO₄^- that was used was .0432M, and the amount that was used was 11.57ml. The mass of the green crystals was .225g.

The teacher converted the Molarity to moles by multiplying it by .01157L. He then converted the moles of MnO₄^- to the moles of oxalate using the formula mentioned aboce.

My question is since this is a titration don't the moles of the MnO₄^- added equal the moles of oxalate at the equivalce point. I don't understand why he used stoichometry to get the moles of oxalate. This is what i had learned in my previos class that the moles of acid or base added equals the moles of base being titrated at the equvialnce point. Isnt the fundamental concept of titration that the moles are equal at equivalance point. I may be missing something here so can some one explain it to me. And if my point of view is worng can some one tell me when that concept of "moles are equal at equivalnce point" is used.

Thanks

 

[Gokul43201]

I am taking chemistry 1A again just to refresh my skills. The teacher was telling us how to figure out the mass composition of oxalate in some crystals that we had just synthesized. The equation is written below

2MnO₄^- + 5C₂O₄^2- + 16H⁺ ----> 2Mn²⁺ + 10CO₂ + 8H₂O

The question is to find the Mass percent of oxalate in the green crystals. The molarity of MnO₄^- that was used was .0432M, and the amount that was used was 11.57ml. The mass of the green crystals was .225g.

The teacher converted the Molarity to moles by multiplying it by .01157L. He then converted the moles of MnO₄^- to the moles of oxalate using the formula mentioned aboce.

My question is since this is a titration don't the moles of the MnO₄^- added equal the moles of oxalate at the equivalce point. I don't understand why he used stoichometry to get the moles of oxalate. This is what i had learned in my previos class that the moles of acid or base added equals the moles of base being titrated at the equvialnce point.

If this is what they taught you before, they were wrong. However, I seriously doubt that any decent Chem teacher would teach the above wrong idea. I strongly suggest you revise the mole concept and understand the theory behind titrations before proceeding.

Isnt the fundamental concept of titration that the moles are equal at equivalance point.

Again, NO !

I may be missing something here so can some one explain it to me. And if my point of view is worng can some one tell me when that concept of "moles are equal at equivalnce point" is used.

This concept is never used .The correct statement is "the number of equivalents of the different species are equal at the equivalence point." In the above example, an equivalent of MnO₄^- has 2 moles and an equivalent of C₂O₄^2- has 5 moles.

You can NOT relate the number of moles of different species WITHOUT using the balanced chemical equation !

 

[babbagee]

I think that was my problem, in the past i never had equations with different coefficients. Usually the equations were already balanced with every thing having a 1 to 1 ratio. I guess that is what messed me up. What you said at is what I meant to say.

Thanks

 

[babbagee]

Would this be correct then, for example if you were doing a acid base titration

n_{base added} = n_{acid initially}
[acid]V_acid=[base]V_base


correct or no
What is the difference between this statement and the statement i made above.

Thanks

 

[Gokul43201]

It is the same as the previous statement, and hence, is incorrect for the same reason.

 

[shrumeo]

This is not an acid/base titration. It is a redox titration.

 

[babbagee]

This is not an acid/base titration. It is a redox titration.

So it is only true for acid and base reactions.

 

[shrumeo]

what is only true for acid/base titrations?
the relation you gave above?
that would only be true for strong, monoprotic acids (HCl, HNO3) reacting with a strong base (NaOH)
if the acid were di or triprotic (H3PO4, H2SO4) then there would be 2-3 inflection points in your titration curve
the pic at the bottom is an example
http://www.dartmouth.edu/~chemlab/techniques/titration.html