[Marija]

Since I'm really bad at analytical chemistry and especially at practical work in laboratory I came across a problem regarding the determination of calcium in a sample of calcium salt. We precipitated calcium ions in a sample with ammonium oxalate and then we solubilized it in sulfuric acid. We titrated the oxalate in this solution with KMnO₄.

The problem is that I have to write down some equations and I'm not quite sure I wrote down the right ones.
First I have to write down the equation for reaction between Ca²⁺ and ammonium oxalate. My idea: Ca²⁺ + (NH₄)₂C₂O₄  CaC₂O₄ + 2NH₄⁺ (Am I missing something? We also added the indicator methyl red and NH₃.)

The second is the equation for reaction of solubilization of calcium oxalate in sulfuric acid. I don't have any ideas what to write. 

The last one is a reaction of oxalate in KMnO₄ in acid environment. I'm sure for this one, so I won't bother you with it.

I'd really appreciated if someone could write down these two equations, because it's important I get it right, 'cause I have to make some calculations afterwards and (of course) I need the right stoichiometry. I have to calculate how much calcium was in our sample.
Oh, and I apologize if my english isn't so good. I sometimes have problems, especially with chemical expressions. 

[Arkcon]

The second is the equation for reaction of solubilization of calcium oxalate in sulfuric acid. I don't have any ideas what to write. 

The last one is a reaction of oxalate in KMnO₄ in acid environment. I'm sure for this one, so I won't bother you with it.

OK, so if you're sure of the last one, you know the products, and the reactants.  And the reactants of the third one, contain the products of the second one.

Can you write, all three reactions, one above the other, without text in between them, and try to figure out what information you have, and what you need to figure out?

[Borek]

Ca²⁺ + (NH₄)₂C₂O₄  CaC₂O₄ + 2NH₄⁺

Not bad, not good either.

Ammonium oxalate is completely dissociated. Go net ionic.

[Marija]

Yes Borek, I can see my mistake now. I had that in my mind, but I was just tiping too fast. You ment: Ca²⁺ + 2NH₄⁺ + C₂O₄^2-  CaC₂O₄ + 2NH₄⁺, right?

Hmm, OK Arkcon, I can try ...

The last one is: 5C₂O₄^2- + 2MnO₄^- + 16H⁺  10CO₂ + 2Mn²⁺ + 8H₂O

So maybe the second one can be: CaC₂O₄ + H₂SO₄  C₂O₄^2- + CaSO₄ + 2H⁺

Am I thinking in a right direction?
Calculations are not a problem, I'm having problems with these reaction and that's why I'm not sure about the stoichiometry.

[Borek]

Yes Borek, I can see my mistake now. I had that in my mind, but I was just tiping too fast. You ment: Ca²⁺ + 2NH₄⁺ + C₂O₄^2-  CaC₂O₄ + 2NH₄⁺, right?

Close, but no banana yet. Remove spectators.

So maybe the second one can be: CaC₂O₄ + H₂SO₄  C₂O₄^2- + CaSO₄ + 2H⁺

Is sulfuric acid dissociated, or not?

[Marija]

Close, but no banana yet. Remove spectators.

Oh, OK. Ca²⁺ + C₂O₄^2-    CaC₂O₄

Is sulfuric acid dissociated, or not?

Damn, same mistake again. Of course it is. Otherwise, the equation is OK?

[Borek]

Is sulfuric acid dissociated, or not?

Damn, same mistake again. Of course it is. Otherwise, the equation is OK?

Not exactly. Oxalic acid is a weak one, so it gets protonated, especially in the presence of the excess strong acid.

[Marija]

Is sulfuric acid dissociated, or not?

Damn, same mistake again. Of course it is. Otherwise, the equation is OK?

Not exactly. Oxalic acid is a weak one, so it gets protonated, especially in the presence of the excess strong acid.

Oh, OK. So I get: CaC₂O₄ + 2H⁺ + SO₄^2-    H₂C₂O₄ + CaSO₄ ? Or am I wrong once again?

[Borek]

This one looks OK to me (even if some nitpickers might argue that it is still not exactly right... ).

[Marija]

Well, thanks for helping.