[Rutherford]

1. Concentrated sulfuric acid was added to the following salts. In which case can a redox reaction not be expected?
a) NaNO₃
b) Na₂S₂O₃
c) NaI
d) Na₃PO₄
e) Na₂C₂O₄
f) NaClO₃
Sulfuric acid could only oxidize species, so I wouldn't expect a reaction for a) and d), but in the answer it is only d). Why?

2. 100 mL of destilled H₂O are mixed with 1 l of ethanol (96 vol-%). The volume contraction is 1%. What is the concentration of water in the mixture?
a) 14.8 mol/L
b) 7.78 mol/L
c) 7.14 mol/L
d) 0.09 mol/L
e) 0.1 mol/L
f) 16.5 mol/L
in 100ml of water there are 100g if I assume a denisity of 1g/ml. This is equal to 5.56mol. When water and ethanol get mixed, the volume is (0.1+1)*0.99= 1.089 L.
c_H2O=5.56/1.089=5.1M, but it is not stated as a possible answer. Where am I wrong?

[Borek]

1 l of ethanol (96 vol-%)

What about other 4%?

[Rutherford]

Thanks for the tip. I assumed that the density is 1g/ml of ethanol solution and got the answer c) which is correct, but the density of 96%-ethanol is actually lower, so I think that the problem is wrong.

What about 1.?

[opsomath]

On the first question, in really concentrated HNO3-H2SO4 mixtures you get the dehydration reaction

O2N-OH + H⁺ -> O2N-OH2+ -> NO2⁺ + H2O.

This can lead to formation of dinitrogen pentoxide, which can then disproportionate into NO2 and O2. In other words, in fuming mixed acid you will probably see a bit of red fumes, which is the NO2.

But the H2SO4 catalyzing a disproportionation reaction seems like an awfully big reach. I wonder what the writer of the question was thinking.

[Rutherford]

That could be it. I now remember that it is the reaction of nitration of benzene. Then the disproporcionation arises.
As this is a competition problem, such questions are quite normal. Thanks for the help.

[opsomath]

That makes sense, it's a bit obscure for a class problem but reasonable for a challenge.