[Rutherford]

This is an IChO problem, and I think that it deserves more attention because it is really challenging, so I opened this topic:


The substance X has been prepared by the following procedures. Copper(II) sulfate pentahydrate (ca 10 g) was dissolved in a mixture of distilled water (80 cm³) and concentrated sulfuric acid (4 cm³). The solution was boiled with analytical-grade metallic tin (10 g) until the solution became colorless and the deposited copper was covered with a grey coating of tin.

The resultant solution was filtered and treated with an ammonia-water solution until the complete precipitation of a product. It was filtered off and washed with water until no odor of ammonia was detectable. The precipitate obtained was added to the nitric acid solution gradually in small portions, with stirring, until the solution was saturated. The suspension was boiled for 2 min, filtered into a warm, insulated flask and allowed to cool slowly. The 1.05 g of crystalline product X was obtained.

Under heating X rapidly decomposes with the mass loss of 17.49%. The residue formed is a binary compound identical with the common mineral of tin. The volatile decomposition products passed over 1.00 g of anhydrous copper(II) sulfate increase its mass by 6.9%.


Attempt:
When the salt was dissolved I ended with Cu²⁺ ions. When tin was added the color disappeared , meaning that Cu²⁺ reduced to Cu⁺. Some copper even reduced to metal Cu and it deposited with a little tin. The rest of the tin was oxidized. The next part I don't understand. Why is ammonia added and then removed with water? Why was nitric acid added-to oxidize Sn²⁺ to Sn⁴⁺ maybe?

[Arkcon]

What chemicals were (possibly) there, and which of them react with ammonia (a strong base and a complexing agent for many metals) then nitric acid ( a proton donor and a mild oxidizing agent) is what you have to work with.  I don't know all that trivia, but they are things for you to work with.

[Rutherford]

Sn²⁺,SO₄^2-,Cu²⁺, Cu⁺ and maybe Sn⁴⁺. Cu2+ reacts with ammonia to form [Cu(NH₃)₄²⁺]. HNO³ maybe oxidizes Sn²⁺ to Sn⁴⁺. Correct?

[Rutherford]

I went to the calculation part:
1.05 g of crystalline product X was obtained. Under heating X rapidly decomposes with the mass loss of 17.49%. The residue formed is a binary compound identical with the common mineral of tin. The volatile decomposition products passed over 1.00 g of anhydrous copper(II) sulfate increase its mass by 6.9%.

X(s) Y(s)+Z(g,l), Y could be identical to cassiterite (SnO₂), so there was 1.05*(1-0.1749)=0.8664g of SnO₂ produced i.e. n=5.75*10^-3mol. If 1 mol of X produces 1 mol of Y, then the molar mass of X is M=1.05/(5.75*10^-3)=182.6/gmol minus 1 atom of Sn, it is 64g/mol. These 64g/mol minus 2 atoms of O give 32g/mol of an unknown group.

There was 0.1836g of Z produced and it is absorbed by 1g of anhydrous copper(II) sulfate. The mass increase is 0.069g, it is not consistent with the mass of Z. Where am I wrong?

[Borek]

Who told you there are only two decomposition products (Y, Z)?

[Rutherford]

If I assume that water is absorbed by CuSO₄ its quantity is n=3.83*10^-3mol, so the mole ratio of SnO₂ and H₂O produced is 3:2, the mass of the other volatile compound is 0.1147.
I assumed that the number of moles of water and the unknown volatile compound are the same, then its molar mass is 30g/mol, it is NO. Compound X is made from 3SnO₂, 2H₂O and 2NO. I wrote (SnO₂)(NO)₂*2H₂O, could this be correct?

[Rutherford]

What important instruction has been omitted in the description of the procedure?