Ions identification (1 Viewer)

[biz2401]

I was given an unknown solution. I followed the Jacaranda table for anions and nothing happen until I put ammonia solution to alkali the solution and it precipitates straight away which is not an expected result. the unknown solution was a colourless solution and it might be Lead presents as cation. Any idea what is happening?

 

[HeroicPandas]

Super EDIT:

Phosphate is insoluble (in a basic medium) with mostly all, besides Group1, ammonium

 

[someth1ng]

You need to give more details about what actually happened and how much NH3 you added - I suppose if there's enough ammonia, you can precipitate out a metal hydroxide.

 

[biz2401]

even one drop of ammonia made it precipitate.

 

[biz2401]

It was lead (II) that presented in the solution and the solution is colourless hence it's very strange if phosphate present as lead phosphate wouldn't dissolve to become a colourless solution like that I think

 

[someth1ng]

When you add ammonia solution:
Copper (II) cation = blue ppt but with excess, it makes a blue solution
Iron (II) cation = same result as test using aqueous NaOH (green ppt)
Iron (III) cation = same result as test using aqueous NaOH (brown ppt)
Calcium cation = No ppt. formed
Aluminium cation = white ppt. (ppt. does not dissolve in excess aqueous ammonia)
Zinc cation = white ppt.
Lead cation = white ppt. (ppt. does not dissolve in excess aqueous ammonia)
Ammonium cation = No ppt. formed. No ammonia gas produced.

Basically, it can be zinc or lead - aluminium probably isn't going be it in HSC and based on previous experience, more likely they will make it lead (HSC).

 

[biz2401]

yes I'm more on lead but why when putting ammonia in it form precipitate with lead?

 

[someth1ng]

NH3(aq)+H2O(aq)-->NH4+(aq)+OH-(aq)
Pb2+(aq)+2OH-(aq)-->Pb(OH)2(s)

I think that's right.

 

[ocatal]

You said anion so I would assume that the unknown anion is phosphate. Phosphate ions precipitate out when the solution is slightly alkaline:
HPO4^2- + H20 <--> H3O^+ +PO4^3-
So when you add ammonia, the hydronium ion concentration is lowered resulting in the equilibrium shifting to the right, producing more phosphate ions. Under standard conditions, this reaction lies well to the left (which is why you don't observe any precipitate being formed before adding ammonia).

 

[salah_01]

so you placed ammonia into a solution that was unknown and a precipitate occured, I think the precipitate is NH4Cl, and the unknown solution contained some compound with Cl- in it or it might contain a phosphate.

 

[HeroicPandas]

so you placed ammonia into a solution that was unknown and a precipitate occured, I think the precipitate is NH4Cl, and the unknown solution contained some compound with Cl- in it or it might contain a phosphate.

All NH4+ (ammonium) compounds are soluble and all Cl- (chlorides) are soluble except for Ag+ (Silver) and Pb2+(Lead - which is slightly soluble)