Consider the two different unknowns, one labeled (a) and one labeled (b), each c
ID: 1019968 • Letter: C
Question
Consider the two different unknowns, one labeled (a) and one labeled (b), each contains ONLY ONE of the two cations/precipitate listed.
Give a single chemical reagent that would allow you distinguish which cation/precipitates is in the unknown.
a) contains either Mg + (aq) or Zn 2+ (aq)....
1) add the following reagent:
2) expected observation for Mg+?...expected observation for Zn 2+?..
b) contains either AgCl(s) or PbCl(s)
1) Add the following reagent:
2) Expected observation for AgCl?...expected observation for PbCl2?
Explanation / Answer
a) Contains either Mg 2+ (aq) or Zn 2+ (aq)....
1) Add the following reagent sodium hydroxide (NaOH)
...expected observation for Mg2+
Mg2+ (aq) + 2OH– (aq) -------> Mg (OH) 2(s)
Mg2+ gives a white precipitate of magnesium hydroxide with sodium hydroxide, which is not soluble in excess of NaOH.
...expected observation for Zn 2+
Zn2+ (aq) + 2OH–(aq) -------> Zn(OH)2(s)
A white precipitate of zinc hydroxide is formed with sodium hydroxide, which dissolves in excess of sodium hydroxide to give a clear colorless solution:
Zn (OH)2(s) + 2OH–(aq) ---------> [Zn(OH)4]2–(aq) (soluble)
b) Contains either AgCl (s) or PbCl (s)
1) Add the following reagent: hot water
2) Expected observation for AgCl- Insoluble in hot water.
Suspend the silver chloride precipitate in 1 mL of water and add 6 M NH3 (aq) dropwise until the precipitate dissolves.
AgCl (s) + 2 NH3 (aq) ------> Ag(NH3)2+ (aq) + Cl-(aq)
Acidify the solution with 6 M HNO3 and the white precipitate should reappear.
Lead (II) chloride can be separated from the AgCl based on its increased solubility at higher temperatures. It is soluble in hot water.
PbCl2 (s) -------> Pb2+ (aq) + 2 Cl-(aq)
The presence of Pb2+ in the aqueous solution can then be confirmed by the formation of a yellow precipitate of PbCrO4 upon the addition of aqueous K2CrO4.
Pb2+ (aq) + CrO42-(aq) PbCrO4 (s) (yellow ppt)
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