[Rutherford]

One of the following statements cannot be correct. State which one.
a) A water-soluble solid contains Mg²⁺, Cr³⁺, and Br^–.
b) A solid soluble in a sodium hydroxide solution contains Al³⁺, K⁺, and SO₄^2-
c) A solid soluble in aqueous ammonia solution contains Ag⁺, Cu²⁺, and Cl^–.
d) A solid soluble in nitric acid contains Ba²⁺, Fe²⁺, and CO₃^2-.
e) A solution neutral to litmus contains Na⁺, Ca²⁺, and PO₄^3-.

Don't understand this. If I take for example a), is it said that there is a solid Mg_xCr_yBr_z that is soluble in water? If yes, how to know if it is?

[Borek]

It can be a solid mixture of MgBr₂ and CrBr₃.

Read all statements, one of them IS wrong.

[Rutherford]

Okay, then a) is true, because both of the compounds you wrote are soluble.
b)unsure, Al³⁺ will be hydrated, now I don't know if it is going to make a precipitate with SO₄^2-;
c)these are soluble in ammonia, so it is true;
d)shouldn't be true, Fe²⁺ could be oxidized and CO₃^2- protonated;
e)isn't neutral because of PO₄^3- basic behavior.
So I got d), e) and I am unsure about b), but only one of these is false. Help needed  .

[Borek]

What is wrong with a solid mixture of FeCO₃ and BaCO₃? You are not asked about what happens when it dissolves, and whether it is possible that it dissolves this or other way, you are asked if such a solid is possible.

[Rutherford]

Then b) is true, too.
BaCO₃ is insoluble in water, but it is probably soluble in HNO₃ because d) has to be right when e) is surely the false one. Thanks for the help.

[Hunter2]

Think again about d. What happens with carbonates dissolved in acid?

[Borek]

Think again about d. What happens with carbonates dissolved in acid?

Sigh. It doesn't matter. Question asks if the solid can exist.

[Hunter2]

It asks: False statement (solids solubility)

d) A solid soluble in nitric acid contains Ba²⁺, Fe2⁺, and CO₃^2-.

My answer: this is wrong, because Carbonate will decompose to carbon dioxide and water and will not exists. Of course it will be dissolved, but in stead of carbonate nitrate will be found.

All others soluble in the given matrix, partly by complexing.

[Borek]

My answer: this is wrong, because Carbonate will decompose to carbon dioxide and water and will not exists.

So what? It doesn't ask about what will be present in the solution, it asks whether the solid exists.

[Hunter2]

Well I would understand the question how I explained, but on second view its also possible how you understood it.

But in this case I would write: A solid soluble in nitric acid contains FeCO₃ and BaCO₃. In solid phase the ions not present.
Probably Iron-II-carbonate is also not existing, because will be oxidized very quick to iron-III compounds.

[Rutherford]

e) is the only false one.
Most of these old IChO problems are very ambiguous, I admit.

[Hunter2]

This is also strange, I thing Calcium phosphate has still some solubility. In neutral solution all ions should be present.

[Rutherford]

The solution can't be neutral to litmus because of the base (PO₄^3-).