[TuAst]

Hi guys
First of all, sorry if I've posted this in the wrong section, I'm new here

I have a (potentially dumb) problem which I hoped you could help me with. It's about the common-ion effect. Here's my line of thought, I would really appreciate it if someone could point out where my logic fails:

1. The common-ion effect states that the solubility of a salt will decrease if more ions common to the ones in the salt are added.
2. The definition of solubility, according to my book, is the concentration of a salt in a saturated solution.
3. According to Le Châtelier's principle, if we add eg Ag+ ions in a AgCl - solution, we will "push" the equilibrium so that more of the salt, AgCl, is produced.
4. So, I can't force it into my head that the concentration of a salt increases while its solubility decreases, seeing as the definition says the concentration of a salt (in a saturated solution) equals the salt's solubility.

If one of you manage to convince me, or tell me where I'm wrong, then you should really be a teacher! It's like trying to learn a monkey physics 

Hope there's someone here that can help me!

TuAst

[DevaDevil]

I can follow your reasoning up to point 4.

2 is wrong though:
you do not look at the "salts" concentration, because when the salt is soluted, it will be in the form of ions. Hence you look at the product of the concentration of the ions. You cannot speak of the "concentration" of the salt as a solid!

In your example of saturated AgCl:

K_sp = [Ag⁺]*[Cl^-]
K_sp is called the solubility product

now what happens if you add concentrated HCl to it (in other words an excess of Cl^- ions):
the product of the ions' concentartion will exceed its K_sp value, and in order to reach the proper product again AgCl will crystallize out. Exactly as le Chatelier's principle predicted.

There will be solid AgCl present, and the product of the ions of that salt will be the same as before the addition of HCl. Solubility did not decrease! And neither did it's "concentration" increase.
 

[TuAst]

Hi DevaDevil

Thank you very much for your answer!
The first part I found very informative and enlightening. (you do not look...You cannot speak of the "concentration" of the salt as a solid!)

But have I understood you correctly if I now think that the common-ions do not decrease the solubility of "the salt". If it is so, I will not trust my chemistry book again. That would be a serious mistake (and I trus a chemist a good deal more than my book).

Thank you for your time!

[Padfoot]

AgCl will crystallize out. Exactly as le Chatelier's principle predicted.

Wouldn't this mean that solubilty of AgCl in this solution has decreased (and not stayed constant)?

[AWK]

The first part I found very informative and enlightening. (you do not look...You cannot speak of the "concentration" of the salt as a solid!)

We are talking about that part od solute that is in soltion, not in the solid state.

1. The common-ion effect states that the solubility of a salt will decrease if more ions common to the ones in the salt are added.

I will not trust my chemistry book again. That would be a serious mistake (and I trus a chemist a good deal more than my book).

Chemistry textbook statement is absolutely correct (within some limits devoted for more advanced chemists) - solubility always decrease taking into account common ion effect only.
The second effect, mainly opposed to common ion effect,  is called "ionic force", and this effect increase solubility of salt greatly.

[Borek]

Let's see, lets say we start with CaSO₄ (pKso = 4.62) and 1L of pure water. Saturated solution will be 4.90x10^-3M, which means 4.90x10^-3 mole (or 0.667 g) was dissolved.

Now we do the same with CaSO₄ and solution that is 1M in SO₄^2-. This time - due to the presence of the common ion - only 2.45x10^-5 mole of CaSO₄ (or (3.35x10^-3g) will dissolve. So obviously solubility is lower. (Note that for clarity we have ignored some other effects here).

I suppose you are confused by the solubility itself - you are most likely trying to incorporate ions already present in the solution into solubility, while you should only be interested in answer to the question: how much solid was transferred to the solution.

[TuAst]

Hi again

Thank you very much guys, your answers were very helpful.
Borek: I liked your example very much, I find that examples are often more declarative than theory.

Thank you all for your help, I might just be bothering you again I suspect