[Borek]

Following five reactions have one common reactant and one common product:

A + HCl F + CrCl₃ + H + I
B + HCl F + G + H + I
C + HCl F + H + I
D + HCl G + H + I
E + HCl H + I

Identify all substances, knowing that C contains 31.83% of metal by mass.

[Rutherford]

Okay, I think it's time to write my attempt:
A=K₂Cr₂O₇
B=KMnO₄
C=KClO₃
D=MnO₂
E=HClO
F=KCl
G=MnCl₂
H=H₂O
I=Cl₂

[Borek]

There is an alternative answer to E.

[Schrödinger]

H₂O₂? Oxidizing HCl to Cl₂ and hence itself getting reduced to H₂O?

[Borek]

Even simpler than that. Reaction I am thinking about was part of an industrial process.

[Rutherford]

E=O₂?

[Borek]

Exactly.

http://en.wikipedia.org/wiki/Deacon_process

[XGen]

If it's not an inconvenience, could you explain your beginning approach when tackling this problem? I'm interested to hear about it

[Borek]

Seeing how in all cases there is a HCl present and a gas is produced can you venture a guess what the gas can be?

[Rutherford]

If it's not an inconvenience, could you explain your beginning approach when tackling this problem? I'm interested to hear about it

Here is the whole approach. First I saw what Borek said, then I started from the last equation and then I went to the first. A is obviously a dichromate and F is a chloride of the metal in the dichromate. C is a chlorate of the metal, and I need only to put in the 31.83%. From the beginning I was thinking about potassium as the metal and then the calculation showed that it is surely potassium (I was thinking about H as water, because it is a common product of redox reactions in acidic solutions and it is present in every reaction in the problem). The rest is easy then.