Video Transcript
What is the correct expression for
the solubility product of silver(I) sulfate?
Silver(I) sulfate has the chemical
formula Ag2SO4. We can deduce this formula since we
are told the metal cation is silver(I). And we are told that the anion is
the sulfate ion, which is SO4 2−.
Silver(I) sulfate is only slightly
soluble in water. When the solid salt is placed in
water, an equilibrium will form with its ions. Some of the salt dissolves, while
some of the ions reform the salt. Not all of the solid dissolves. In fact, only a small portion of
the salt goes into solution to form separate ions, and that is why we say it is
slightly soluble.
As an aside, we sometimes say
silver(I) sulfate is insoluble, but in reality, no compound is completely
insoluble. All compounds dissolve to some
degree, even if it is only slightly.
We are asked to write the
expression for the solubility product of this slightly soluble salt. We can write the solubility product
constant, or 𝐾 sp, expression as follows. 𝐾 sp is equal to the concentration
of the ions in solution multiplied by each other. We mustn’t forget to raise each
term to the power of the stoichiometric coefficient from the balanced equation. Of course, this power of one is
implied, so we can leave it off.
Now, usually an equilibrium
constant has the concentrations of the products in the numerator and the
concentrations of the reactants in the denominator. But since the reactant, silver(I)
sulfate, is a solid and because it is so sparingly soluble, we can essentially
consider its concentration to be unchanging. For this reason, for 𝐾 sp
expressions, we leave out the reactant concentration and do not have a term in the
denominator.
Finally, what is the correct
expression for the solubility product of silver(I) sulfate? The answer is 𝐾 sp is equal to the
concentration of Ag+ squared multiplied by the concentration of SO4 2−.
