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Aqueous solution problem

  • #1
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Homework Statement



A sample of an aqueous solution of sodium halides is added to a sample of an aqueous solution of silver nitrate, resulting in a precipitation. To the mixture containing the precipitation was added a sample of an aqueous solution with ammonium hydroxide, dissolving the precipitation partially. To the remainder of the precipitation was added a sample of an aqueous solution concentrated with ammonium hydroxide, dissolving the precipitation partially again.

If the solution containing the sodium halides is made up of NaF, NaCl, NaBr and NaI, which silver halide(s) are not dissolved from the precipitation?

Homework Equations





The Attempt at a Solution



What is the first reaction I should consider in these types of problems?
[tex] NaCl_{(aq)} \rightarrow Na^++Cl^-[/tex]
or
[tex] NaCl_{(aq)}+AgNO_{3(aq)} \rightarrow AgCl+\ ? [/tex]
I'm quite honestly lost. Should I just know stuff? Or is there some kind of logic?
 

Answers and Replies

  • #2
SteamKing
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In chemistry, the more you know, the better off you are. You can always google 'silver' to learn more about the chemistry of its compounds.
 
  • #3
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As per my knowledge, adding ammonium hydroxide will lead to the following complexation reaction:
[tex]Ag^+\,+ \, 2NH_3 \rightarrow [Ag(NH_3)_2]^+[/tex]

I guess you need to solve the problem on the basis of solubility products of the silver halides but I am not sure about this. I wait for someone to comment on this.
 
  • #4
Borek
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I guess you need to solve the problem on the basis of solubility products of the silver halides
combined with a stability constant for the diammine silver complex.
 
  • #5
epenguin
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combined with a stability constant for the diammine silver complex.
I remember we had that one recently.
How much AgBr could dissolve in 1.0 L of 0.40 M NH3? Assume that Ag(NH3)2+ is the only complex formed. [Kf=1*108 ; Ksp=5*10-13]
Well obviously no one is going to remember the figures, but useful qualitative information to retain would be the comparative solubility of the different silver salts, and its explanation.
 
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Last edited by a moderator:
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In chemistry, the more you know, the better off you are.
Yeah, but how much of chemistry am I gonna be able to work out? When I see people solving chemistry problems, they just use facts. There are a lot of charts and tables, but very little logic to follow. Am I tackling chemistry wrong? If I simply don't know what happens to sodium halides and silver nitrate when mixed in water; should I just give up or is there a set of axioms that I could use to figure it out? Is problem solving in chemistry just using previous attained facts?

EDIT: Also, for chemistry, if I have a limited amount of time to study for a test, should I just read a lot on the test subjects or should I insist in solving problems? Because if each problem is only solvable by knowing facts, I feel like solving one problem won't help me to solving others. Like mathematics where practicing problem solving goes a long way.
 
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  • #8
epenguin
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If I simply don't know what happens to sodium halides and silver nitrate when mixed in water; should I just give up or is there a set of axioms that I could use to figure it out? Is problem solving in chemistry just using previous attained facts?
What that suggests is your problem which would not be your fault is that you have not spent much time in the laboratory or even seeing laboratory demonstrations. It is common to need to detect or assay chloride, and from having seen it enough times the high insolubility of silver halides is something I still easily remember, as I do the white colour of AgCl, and pale yellow colour of AgBr, and I think redissolving them in ammonia.

I think chemistry often struggled for its place at school and I wonder if it isn't now being pushed further aside, and only its indispensibility for biology keeps it there. But unlike other subjects which can still be learned later, what you don't see at school you probably never will.

It does not have the same almost deductive style in which physics can be put, but it is a science nevertheless, but has more rationale than the "stamp collecting" dismissals and you need to seek all the rationalisations and hopefully will find a strange horrible fascination :biggrin: in them; it also needs the laboratory though and you cannot do everything from a book as at a pinch you could physics.

Added: Also take every opportunity to connect up things. This has just brought to mind the "silver mirror test" for aldehydes. It consists of reduction of Ag+ to Ag. Or rather reduction of the ammonia complex, for it is done in the presence of ammonia. And why - why not use silver nitrate which is perfectly soluble? I either did not remember, or never knew or questioned. But here is the explanation:

The half-equations indicate that ammonia forms a complex with the silver ion, which is more difficult to reduce than the silver ion. This is because silver ions form more stable complexes with NH3 than with water.

"If silver nitrate is used without ammonia, the silver ion is reduced so quickly that colloidal silver metal would appear. The solution would become a black, cloudy liquid."
http://www.rsc.org/Education/EiC/issues/2007Jan/ExhibitionChemistry.asp
 
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  • #9
Borek
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If I simply don't know what happens to sodium halides and silver nitrate when mixed in water; should I just give up or is there a set of axioms that I could use to figure it out?
Solubility rules.
 

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