We are doing a lab where we test an unknown sol. for 3 of the 10 possible ions: Ag+,Pb2+,Ca2+,Cu2+,Fe3+,Al3+,Zn2+,Mg2+,Na+,k+
I elimanted the possibility of Ag+,Pb2+,Ca2+,Na+ through a flame test,usinng Na2So4 and HCl
I believe I have Na+ through flame test
When adding NaOH to the unknown sol.(with no other sol. previously added) I see a gelatinous precipitate that is kinda clear/white. This must mean there is no ,Cu2+,Fe3+ because a colored precipitate would have formed.
However the gelatinous precipitate then dissolves.
Could this mean it's Al3+ or Zn2+ because they form a clear complex compound with excess NaOH?
How can I further test to be sure of the ions in the solution?
Any help is appreciated. Thanks
Adding 1M NaOH into an unknown solution containing 2 of the three ions: Al3+,Zn2+,Mg2+?
2016-05-24 6:57 pm
回答 (2)
2016-05-24 7:12 pm
✔ 最佳答案
It's possible the alumniumcomplex. Although quite some ions form insoluble hydroxides, amfoteric substances form soluble hydroxocomplexes. Aluminium can be checked for by adding some drops of aluminon. It should form a dark red 'lake'.
2016-05-25 2:46 pm
You have listed only 9 ions only, i.e. Ag⁺, Pb²⁺, Ca²⁺, Cu²⁺, Fe³⁺, Al³⁺, Zn²⁺, Mg²⁺ and Na⁺.
If the solution contains 2 of the three ions of Al³⁺, Zn²⁺ and Mg²⁺, they would be Al³⁺ and Zn²⁺ ions.
All of Al³⁺, Zn²⁺ and Mg²⁺ can react with sodium hydroxide solution to form white precipitate.
Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)
Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)₂(s)
Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)
Among these three insoluble hydroxide, only Al(OH)₃ and Zn(OH)₂ re-dissolves in an excess of sodium hydroxide solution.
Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq)
Zn(OH)₂(s) + 2OH⁻(aq) → [Zn(OH)₄]⁻(aq)
It can be deduced that the 2 ions are Al³⁺ and Zn²⁺.
The test you have done is sufficient to prove that the 3 ions are Na⁺, Zn²⁺ and Al³⁺ ions.
If the solution contains 2 of the three ions of Al³⁺, Zn²⁺ and Mg²⁺, they would be Al³⁺ and Zn²⁺ ions.
All of Al³⁺, Zn²⁺ and Mg²⁺ can react with sodium hydroxide solution to form white precipitate.
Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)
Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)₂(s)
Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)
Among these three insoluble hydroxide, only Al(OH)₃ and Zn(OH)₂ re-dissolves in an excess of sodium hydroxide solution.
Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq)
Zn(OH)₂(s) + 2OH⁻(aq) → [Zn(OH)₄]⁻(aq)
It can be deduced that the 2 ions are Al³⁺ and Zn²⁺.
The test you have done is sufficient to prove that the 3 ions are Na⁺, Zn²⁺ and Al³⁺ ions.
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