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Redox Titration Question - Answer Check & Questions

  • #1
Good day!

I would just like to confirm a couple things in regards to my solution for this problem.

Problem:
A sample of hydrated ethanedioic acid crystals (formula H2C2O4 . 2H2O) of mass 0.2145g was dissolved in water and the solution was used to standardise some potassium permanganate solution by titration in acidic conditions. 35mL of the KMnO4 solution were required to react completely with the ethanedioic acid solution. Calculate the moles of ethanedioate ion present in the solution of the acid crystals.

Background Information:
Oxidation of ethanedioate ions: C2O42- -> CO2
Reduction of permanganate in acid: MnO4- -> Mn2+

Solution:
First balance the equation.
6H+ + 5H2C2O4.2H2O + 2KMnO4 -> 10CO2 + 2Mn2+ + 2K+ 18H2O

(88.02g/mol C2O42-) / (126.08g/mol H2C2O4.2H2O) = (x) / (0.2145g H2C2O4.2H2O)

x = 0.149748493g 2O42-

Molar mass of 2O42- = 88.02g/mol
Mass of C2O42- = 0.149748493g
Number of Moles = 0.0017 mol C2O42-

Final answer: 0.0017 mol

Question
Is that the correct answer?

I was thinking if I need to multiply that 88.02g/mol and 126.08g/mol in that ratio by 5, because the balanced equation states there should be 5 moles of the hydrate used within that reaction. However, in this case it would not matter cause the two would cancel having been in both the numerator and the denominator. But technically, was the coefficient "5" needed in proper calculation? Or does it not matter?

Secondly, when I say H2C2O4 . 2H2O, there is 2 + 4 = 6 Hydrogen atoms, but when I put a coefficient in front of the hydrate, say 2, 2H2C2O4 . 2H2O, the 2 should apply to the whole hydrate including the H2O correct? So there should be 2(2) + 2(4) = 12 Hydrogen atoms, NOT 2(2) + 4 = 8. Can anyone please confirm this?

Your help is always appreciated, and I thank you in advance!
 

Answers and Replies

  • #2
Borek
Mentor
28,303
2,687
Ignore the reaction, it doesn't matter. All that matters is mass of the acid and its molar mass. Compare your question to

A sample of hydrated ethanedioic acid crystals (formula H2C2O4 . 2H2O) of mass 0.2145g was dissolved in water. Calculate the moles of ethanedioate ion present in the solution of the acid crystals.

Is it in any way different?
 

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