Question on Solubility (1 Viewer)

[Dreamerish*~]

This's from last year's trials paper of my school.

In the laboratory a test solution was analysed. It formed a pale yellow precipitate with silver nitrate solution. When sodium hydroxide was added to the solution it formed a blue precipitate which dissolved when excess sodium hydroxide was added. Deduce the ions present in the test solution and write suitable equations.

What I thought was:

PO₄^3-_(aq) + 3AgNO_3(aq) → Ag₃PO_4(s) + 3NO₃^- ... which forms a yellow precipitate.

Cu²⁺_(aq) + 2NaOH_(aq) → Cu(OH)_2(s) + 2Na²⁺_(aq) ... which forms a blue precipitate.

But Cu₃(PO₄)₂ is a precipitate itself, so how can it exist as a solution?
Also, why does the blue precipitate disappear after adding excess NaOH?

 

[Alimoe_KG]

ditto.

eco > chem.

 

[Dreamerish*~]

ditto.

eco > chem.

GET OUT OF MY THREAD

 

[Alimoe_KG]

Air is everywhere.

 

[nit]

I'm not entirely sure if your question is right. There probably should be some ammonia in it.

Also, the pale yellow ppte is probably due to the formation of silver iodide, or bromide. So the counterion is probably I- - that's generally taken to be pale yellow in colour.

As for the cation, Cu2+ is a good bet. The property of redissolving in excess hydroxide is due to amphoteric behaviour - ie the ability to act as an acid or as a base. This involves the concept of lewis acids and bases (ie electron pair acceptors and donors respectively). When ammonia is added to copper(II) solutions, a light blue precipitate of copper (II) hydroxide is formed. When excess ammonia is added, the ammonia ligands complex the copper cations to form a progressively darker tetraammine copper (II) coordination complex.

The main amphoteric hydroxides formed using sodium hydroxide are aluminium, lead and zinc, all of which have white, gelatinous hydroxides. Hence I think the question may be wrong.

 

[Dreamerish*~]

I'm not entirely sure if your question is right. There probably should be some ammonia in it.

Yeah, that's what i thought too, because Cu(OH)₂ dissolves in NH₃ to form a dark blue solution (I think).

I wrote that in my answer, but the question mentioned nothing about ammonia, and said that when sodium hydroxide was added, the precipitate disappeared.

 

[xiao1985]

@ dreamish: knowing fort, i say the cation present is Cr3+, reference is pp270 cchsc
table 8.3

 

[nit]

well if it is chromium (III), then in that case the precipitate ought to be green. Chromium(III) hydroxide is amphoteric however.

I don't think chromium(II) hydroxide is amphoteric - it isn't highly charged enough. That does satisfy the blue colour though.

 

[Dreamerish*~]

@ dreamish: knowing fort, i say the cation present is Cr3+, reference is pp270 cchsc
table 8.3

thank you!

 

[Abtari]

are we supposed to know the solubility rules off by heart?

what other types of short/long answer questions can they ask on ions and precipitates/flame tests/ etc..

 

[Dreamerish*~]

are we supposed to know the solubility rules off by heart?

what other types of short/long answer questions can they ask on ions and precipitates/flame tests/ etc..

Yes we are, unfortunately.

We'll probably be asked questions which require us to distinguish between two elements by flame test or precipitate.

 

[KFunk]

I am yet to learn those solubility rules... I should probably get onto it. Is there a definitive list of which solubility rules/flame tests/flow charts we need to memorise or is it up in the air a bit?

 

[Pace_T]

Does anyone know where I can get the solubility rules? or is anyone willing to give me the charts/information that I have to remember?
My textbooks don't have any decent information on this, and I don't have conquering chemistry either.

Thanks in advance.

 

[Dreamerish*~]

Solubility rules are a bit messy, because there are so many exceptions. This's the best I can do. I looked in C.C. but a lot of the ions listed there are uncommon.


SOLUBILITY AND INSOLUBILITY

All nitrates are soluble.
All chlorides are soluble except Ag⁺, Pb²⁺.
All sulphates are soluble except Ba²⁺, Pb²⁺. (slightly soluble Ca²⁺, Ag⁺)
All carbonates are insoluble except Group I and NH₄⁺.
All hydroxides are insoluble except Group I, NH₄⁺, Ba²⁺.
All phosphates are insoluble except Group I and NH₄⁺.


TEST FOR SPECIFIC CATIONS

Ba²⁺: forms white precipitate with SO₄^2-. Apple-green in flame test.
Ca²⁺: forms white precipitate with SO₄^2-. Brick-red in flame test.
Pb²⁺: forms yellow precipitate with I^-, and white precipitate with SO₄^2-. Not a good idea to test by flame as lead fumes are poisonous.
Cu²⁺: forms blue-green precipitate with OH^-. Aqua in flame test.
Fe²⁺: forms green precipitate with OH^-.
Fe³⁺:forms brown precipitate with OH^-.
Ag⁺: forms white precipitate with Cl^-.

Note: the only cations that would require the flame test to be distinguished from each other are Ba²⁺ and Ca²⁺ as they both form white precipitates. Therefore make sure you remember them.


TEST FOR SPECIFIC ANIONS

CO₃^2-: add HCl, and observe effervescence of colourless gas (CO₂), confirmed by lime water.
Cl^-: add AgNO₃, white precipitate formed.
SO₄^2-: add drops of Ba(NO₃)₂, white precipitate formed.
PO₄^3-: add Ag(NO₃), brown precipitate formed (Ag₃PO₄). OR add drops of Ca(NO₃)₂, white precipitate formed (Ca₃(PO₄)₂)).


Hope this helps.

 

[Pace_T]

Thank you very much, Dreamerish*~.

 

[Pace_T]

Oh, just one more thing
Say you are testing for a anion, and you add Fe(III) and Fe(II) ions (separately)
what actual compounds are being added? If there's many, what are some possible ones?
*Just thought this would be important incase if they ask us to identify an anion. I don't think it's correct to say to add 'Fe3+ ions' - or is it?
Cheers.

 

[Dreamerish*~]

Oh, just one more thing
Say you are testing for a anion, and you add Fe(III) and Fe(II) ions (separately)
what actual compounds are being added? If there's many, what are some possible ones?
*Just thought this would be important incase if they ask us to identify an anion. I don't think it's correct to say to add 'Fe3+ ions' - or is it?
Cheers.

It is okay to say "add Fe³⁺ ions", I'm pretty sure. But to be more accurate, you should state which compound to add.

When you're adding say, Ag⁺ ions to test for the presence of Cl^-, the Ag⁺ compound has to be a solution, of course - you can't be adding a solid!

To be safe, always use NO₃^-. As stated in my previous post, it's soluble in everything.

Hence you would say, "add AgNO₃ solution to the Cl_- solution".

 

[Pace_T]

It is okay to say "add Fe³⁺ ions", I'm pretty sure. But to be more accurate, you should state which compound to add.

When you're adding say, Ag⁺ ions to test for the presence of Cl^-, the Ag⁺ compound has to be a solution, of course - you can't be adding a solid!

To be safe, always use NO₃^-. As stated in my previous post, it's soluble in everything.

Hence you would say, "add AgNO₃ solution to the Cl_- solution".

So you're saying I could add Fe(NO₃)₂ and Fe(NO₃)₃ for Fe²⁺ and Fe³⁺, respectively to detect an anion? (assuming that it would be insoluble with Fe(II) and Fe(III))
Thanks for your help Dreamerish*~. You're so helpful

 

[Dreamerish*~]

So you're saying I could add Fe(NO₃)₂ and Fe(NO₃)₃ for Fe²⁺ and Fe³⁺, respectively to detect an anion? (assuming that it would be insoluble with Fe(II) and Fe(III))
Thanks for your help Dreamerish*~. You're so helpful

Yes, that would be right. NO₃^- forms no precipitates with anything.

No worries, good luck!