[skippys]

I'm having some trouble with this question:

Consider the following reaction in which solid silver chloride is in equilibrium with silver and chloride ions:

AgCl(s)  Ag⁺(aq) + Cl^-(aq)

If we add 0.05 M of NaCl to this solution, the _______ of Ag⁺(aq) will decrease (Choose all correct answers)

1. activity
2. solubility
3. activity coefficient
4. concentration

I thought the answer was solubility and concentration because adding NaCl causes the common ion effect, but apparently that's not right. I also tried just concentration and that was wrong too. Help?

[Borek]

You were right about concentration, it will change for at least two reasons.

You can't speak of a solubility of a single ion.

You have completely ignored activity and activity coefficient - what do they depend on?

[skippys]

Ksp must remain constant so if concentration decreases, then the activity coefficient must increase. Since the activity coefficient increases, then wouldn't activity also increase? Unless you're taking into account the decreased concentration, but in that case it would depend on whether the activity coefficient or concentration has a larger effect.

[Borek]

Ksp must remain constant so if concentration decreases, then the activity coefficient must increase.

That's not how it works, but it is so wrong I even don't know where to start

Hint: there is an important parameter describing solution containing different ions. This parameter is used to calculate activity coefficients in the Debye-Huckel theory.

If you don't know what I am talking about, read about the DH theory. If you are asked a question about activity and activity coefficients, you are expected to know at least the most simplified version.