Question

Suppose you have an unknown clear, colorless solution. In attempt to identify it, you add NaOH dropwise and find that a white gelatinous precipitate forms, but when more NaOH was added the precipitate dissolved again. To another test tube of the same unknown solution is added AgNO3 dropwise, and a white precipitate forms. What salt most likely comprises the unknown solution? Explain your reasoning (3 points) 2. Suppose you have a different unknown solution that you know to be 50/50 mixture of 2 different 1 M salt chloride solutions. In attempt to identify it, you add sodium hydroxide dropwise, and the solution forms a blue precipitate. Furthermore, upon heating the solution slightly, you notice the solution starting to bubble (below the boiling point water). What are the two likely cations in this unknown solution? Explain your reasoning (3 points).

Answer 1

1. If NaOH is added to the solution drop wise and a white gelatinous precipitate forms then that is due to presense of metal ion Al3+. Aluminium forms a white precipitate Al(OH)3 in presense of sodium hydroxide. Now if excess NaOH is added the precipitate dissolves due to the formation of Al(OH)4—​​​​​​. This is a test for confirmation of Al3+ Cation.

To another test tube of the same unknown solution when AgNO3 is added a white precipitate forms due to the formation of insoluble silver compounds. Cl- forms insoluble compound with Ag+. This is a test for confirmation of Cl- Anion.

Ag+ + Cl- → AgCl(s).

Thus the salt present in this solution is Aluminium Chloride (AlCl3).

2. We have two salt chlorides mixtures and in one test tube of this unknown solution we add sodium hydroxide drop wise and a blue precipitate forms. This is due to the presense of metal ion Cu2+.​​​​​​ Copper(II) chloride forms a blue precipitate of Cu(OH) ​​​​2 in presense of sodium hydroxide.

Now when this solution is heated below the boiling point, bubbles are formed. The salt chloride that is also present in this solution is ammonium chloride. Ammonium chloride when heated in presense of sodium hydroxide, it gives bubbles(ammonia gas).

NH4Cl + NaOH - - - - > NH​​​3+ H2O + NaCl.

The two cations present are Ag+ and NH4+.