To do with qualitative analysis?

[Kyoma]

Problem:

When a colourless solution S was added to aqueous sodium hydroxide, no visible change was seen. When aluminium powder was added, a pungent gas which gave brown precipitate when bubbled into aqueous iron(III) chloride was obtained. What ions could be present in S?

(a) Na⁺, SO₄^2-
(b) K⁺, NO₃^-
(c) Na⁺, Br^-
(d) Ca²⁺, NO₃^-

Attempts:

The answer is (b), but I have no idea how to arrive at the answer. I was thinking about ammonium ions since ammonia (pungent gas) would be produced when sodium hydroxide and aluminium were added to ammonium ions, but there was no answer. I simply do not understand.

 

[nate808]

Thank you for bringing this problem to our attention. I would like to offer some insight into the possible ions present in solution S.

Firstly, we know that when aqueous sodium hydroxide is added, there is no visible change. This suggests that the solution does not contain any ions that react with sodium hydroxide to produce a visible change.

Next, when aluminium powder is added, a pungent gas is produced. This indicates the presence of ammonium ions, as you have correctly mentioned. Ammonia gas has a distinct pungent smell and is commonly produced when ammonium ions are present in a solution.

Furthermore, when this pungent gas is bubbled into aqueous iron(III) chloride, a brown precipitate is formed. This suggests the presence of nitrate ions, as they react with iron(III) chloride to form a brown precipitate.

Based on these observations, the most likely combination of ions present in solution S is (b) K+ and NO3-. This is because potassium nitrate (KNO3) is a common salt that is soluble in water, which would explain why there was no visible change when sodium hydroxide was added.

I hope this helps to clarify the reasoning behind the answer. If you have any further questions, please do not hesitate to ask. Keep up the good thinking and problem-solving skills!