Chemistry question! they are so hard!! please! 15points!

1.
This is because cations of alkali metals and ammonium ion would not react with the anion(s) in the unknown solutions.

Example
It is a common method to use a solution of Cl-(aq) ion to test for Ag+(aq) ion. If Ag+(aq) is presence in the unknown solution, a white precipitate of AgCl is formed.

Now, an unknown solution is given, which is actually MgSO4 solution. You are asked to test whether the solution contains Ag+(aq) ion or not. Actually, the answer is: not.

If BaCl2 solution is used instead as the reagent in the test, Ba2+(aq) ion would give a white precipitate with SO42-(aq).
Ba2+(aq) + SO42-(aq) → BaSO4(s)
Due to the formation of a white precipitate, an incorrect conclusion would be drawn that the unknown contains Ag+(aq) ion.

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2.
This is because nitrate ion would not react with the cation(s) in the unknown solutions.

Example
It is a common method to use an acidified solution of Ba2+(aq) ion to test for SO42-(aq) ion. If SO42-(aq) is presence in the unknown solution, a white precipitate of BaSO4 is formed.

Now, an unknown solution is given, which is actually Pb(NO3)2 solution. You are asked to test whether the solution contains SO42-(aq) ion or not. Actually, the answer is: not.

If acidified (with HCl) BaCl2 solution is used instead as the reagent in the test, Cl-(aq) ion would give a white precipitate with Pb2+(aq).
Pb2+(aq) + 2Cl-(aq) → PbCl2(s)
Due to the formation of a white precipitate, an incorrect conclusion would be drawn that the unknown contains SO42-(aq) ion.
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