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Topic: 3 step reaction sequence involving redox  (Read 2536 times)

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Offline Augh

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3 step reaction sequence involving redox
« on: October 17, 2012, 05:25:06 PM »
I've been with this problem for a while, really have no idea how I should tackle it:

I've been facing this for a while, no idea how to tackle it:

"The standard laboratory technique for determination of the concentration of calcium in blood serum and urine uses the following reaction sequence involving ammonium oxalate, (NH4)C2O4, and potassium permanganate, KMnO4.

Ca2+ + (NH4)C2O4 = CaC2O4(s) + 2NH4+ (1)
CaC2O4(S) + H2SO4 = H2C2O4 + CaSO4 (2)
KMnO4 + H2C2O4 + H2SO4 = K2SO4 + MnSO4 + CO2 + H2O (3)

A 24 hour urine sample is collected from an adult patient, reduced to a small volume, and titrated with 26.3 mL of 0.0946M KMnO4 solution. How many grams of calcium oxalate are in the sample? Normal range for Ca2+ output for an adult is 100 to 300 mg per 24 hours. Is the sample within normal range?

For the purpose of balancing, the redox reaction (3) can be simplified as follows:
MnO4- + H2C2O4 = Mn2+ + CO2        "

I know it's long, but really, any help here would be greatly appreciated.

Offline Borek

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Re: 3 step reaction sequence involving redox
« Reply #1 on: October 17, 2012, 05:42:42 PM »
What is molar ratio of number of moles of oxalic acid to the number of moles of calcium?

Can you think of a way of calculating number of moles of oxalic acid from the stoichiometry of the reaction with the permanganate?
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Offline Augh

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Re: 3 step reaction sequence involving redox
« Reply #2 on: October 17, 2012, 06:26:13 PM »
If the molar ratios for both Oxalic acid and Potasssium permanganate are 1 in the last reaction, we can find the number of moles of permanganate used, the use that to find how many moles of acid there were originally (Question about this, because oxalic acid has 2 H, would it require 2 moles of permanganate to neutralize 1 mole of it?)

And if we have the moles for Oxalic acid, the molar ratios apply through the entire sequence, so we have the moles of Oxalate and Calcium, right?

Offline Borek

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Re: 3 step reaction sequence involving redox
« Reply #3 on: October 18, 2012, 03:26:13 AM »
Note: reaction between permanganate and oxalic acid is not 1:1, nor 2:1 - you need to balance the reaction to find the ration, and it is not the neutralization. But you are given nice skeletal reaction, that should help you. Also note even if the molar ratios are not 1:1 you can still do the calculations - just the ratio will be different. It is all about stoichiometry.

You are right about the second part - yes, you have to follow molar ratios through the whole sequence. In this particular case it is quite easy, it doesn't have to be that easy always. The only harder part here is the permanganate/oxalic acid reaction.
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