In which of the given solutions with EQUAL concentrations will silver bromide be LEAST soluble?

a. NH_3(aq)
b. NaNO_3(aq)(aq)
c. NaBr(aq)
d. Ca(NO_3)_2(aq)

1 Answer
Sep 14, 2015

The answer is indeed (c), NaBr, due to the common ion effect. The common ion is bromide, which restricts solubility of the silver ion.

Explanation:

The solubility of silver halides in water is another equilibrium reaction, one that we treat in the normal way:

AgBr(s) rightleftharpoons Ag^+ + Br^(-); K_(sp) = ?

Note that the reactant, AgBr(s), does not appear in the equilibrium expression because as a solid it has no concentration. The equilibrium constant, K_(sp), is simply a number and is considered dimensionless. We don't know what it is, but it is very small. Given that it is an equilibrium constant, we can write:

K_(sp) = ? = [Ag^+][Br^-]

This relationship governs the solubility of silver bromide in water. If [Ag^+][Br^-] > K_(sp), then silver bromide, AgBr, will precipitate until [Ag^+][Br^-] = K_(sp).

Please do not be intimidated by the length of this answer. The crucial bit of information governing the reaction is the rxn,

AgBr(s) rightleftharpoons Ag^+ + Br^(-); K_(sp) = ?

If the ion product [Ag^+][Br^-] is above a certain number, precipitation of the silver salt will occur. If we introduce ammonia, NH_3 to the reaction mixture, which can form the soluble ion Ag(NH_3)_2^+, do you think that some of the AgBr precipitate will go up?

I take it this is a 1st year chemistry question?