[Rutherford]

Substance A is an anhydrous purple potassium salt. Substance B is the salt of one cation and one anion which in reaction with BaCl₂ gives a white precipitate insoluble in dilute acid. When 7.11g of substance A reacts with B in excess in neutral environment, 9.79g of a dark brown oxide is obtained. Determine A and B.

A is certainly KMnO₄. 0.045mol of it reacted to form the oxide. The anion in B is SO₄^2-, but here I am stuck. I don't have the reaction equation. What to do now?


Also one quick question: Why is the formula Al(HSO₄)₃ incorrect?

[Borek]

Have you noticed 9.79 g is more than 7.11 g? It tells you something.

[Rutherford]

The molar mass of the oxide is bigger than that of KMnO₄. I can't think of anything else  .

[Borek]

What is the most likely identity of the brown oxide?

[Rutherford]

Of manganese probably, and the one that has the biggest molar mass is Mn₂O₇, so what now?

[Borek]

No, it is not Mn₂O₇, and it is quite unlikely product.

What is the most likely identity of a brown oxide that is a product of permanganate reaction?

[Rutherford]

MnO₂ is usually produces in neutral environment so it could be the oxide. It's amount is 0.1125mol, and the amount of permanganate is 0.045mol, so I don't have enough manganese in the permanganate meaning that B is MnSO₄. Using the mass data, I got the following mole ratio: n_KMnO4:n_MnSO4:n_MnO2=2:3:5. The ratio is in agreement with the redox balancing, so the reaction is:
2KMnO₄+3MnSO₄+2H₂O 5MnO₂+K₂SO₄+2H₂SO₄

I think that this is okay now. Thanks for the help.