[Rutherford]

0.6 moles of an unknown compound was oxidized with an aqueous solution of KMnO₄. The following compounds were obtained: MnO₂ (n=0.4mol), KHCO₃ (n=0.4mol), CO₂ (n=0.2mol) and water. What was the unknown compound?

I came with this equation:
3X+2KMnO₄ 2MnO₂+2KHCO₃+CO₂+yH₂O
n of water is unknown, so I got stuck here. How to determine X now?

[Borek]

How many carbon atoms in a molecule of X? What are possible answers?

[Rutherford]

1 C atom, and know I have to guess and write the equations for alcohol, aldehyde and acid, and the only one that can be balanced is the the one with the acid, so the answer is HCOOH. Thanks for the help.

[AWK]

Better use equation in the form:
n(C₃H_zO_t+2KMnO₄ = 2MnO₂+2KHCO₃+CO₂+yH₂O) n =1, 2, 3 ...
and express z and t by y.
You will get 2 equation with 3 variables and start modeling: y =1, 2, 3 …. For n=1, then 2 and so on. For each formula check the multiplicity (eg. C₃H₆O₆ = 3CH₂O₂) and possibility of existence such a compound (CH₂O₂ exists, C₃H₆O₆ exists but is rather unstable).

[Rutherford]

Thanks for the suggestion, but that's the same if I assume that CH₂O₂ is the empirical formula, then I can just multiply the numbers in the subscript by 2,3,4...

[AWK]

There are more possibilities than simple multiplicity of formic acid formula!

[Rutherford]

Yes, you are right. I understood now.

[AWK]

HCOOH is the only solution of your problem. I show you how to solve more general problem limited to molar ratio of permanganate, carbon dioxide and hydrogen carbonate.