Ksp: The Solubility-Product Constant


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This principle was first put forth by Walther Nernst in 1899. It has to do with solid substances usually considered insoluble in water. In each case, we will consider a saturated solution of the insoluble substance that is in contact with some undissolved solid. Important points to consider are:

1) Some of the solid does dissolve. Not very much, but enough.
2) The substance dissociates upon dissolving.
3) There exists an equilibrium between the undissolved solid and the solvated ions.

Since equilibrium principles can be used, that is where we start. Our first example is silver chloride, AgCl. When it dissolves, it dissociates like this:

AgCl (s) <===> Ag+ (aq) + Cl¯ (aq)

An equilibrium expression can be written:

Kc = ( [Ag+] [Cl¯] ) / [AgCl]

Now, we come to an important point. When the AgCl is enclosed in square brackets like this -- [AgCl] -- that means the "molar concentration" of solid AgCl. This value is a constant!! Why?

Answer: The "molar concentration" of a solid (it's not a useful chemistry idea, so it is seldom used) can be directly related to the density, which is also a constant. Here is a graphic which summarizes the relationship:

What we do is move the [AgCl] to the other side and incorporate it with the equilibrium constant.

Kc [AgCl] = [Ag+] [Cl¯]

Since Kc [AgCl] is a constant, we replace it with a single symbol. Like this:

Ksp = [Ag+] [Cl¯]

(Just a side point - as you go on in chemistry, you'll get introduced to the concept of activity. The activity of a solid is defined as equal to the value of one. Since the activity of AgCl(s) = 1, it just drops out. However, like I said, activity is for the future. Not right now.)

It turns out that the Ksp value can be either directly measured or calculated from other experimental data. Knowing the Ksp, we can calculate the solubility of the substance in a very straightforward fashion.

Here are several other examples of dissociation equations and their Ksp expressions:

Sn(OH)2 (s) <===> Sn2+ (aq) + 2 OH¯ (aq) Ksp = [Sn2+] [OH¯]2
Ag2CrO4 (s) <===> 2 Ag+ (aq) + CrO42¯ (aq) Ksp = [Ag+]2 [CrO42¯]
Fe(OH)3 (s) <===> Fe3+ (aq) + 3 OH¯ (aq) Ksp = [Fe3+] [OH¯]3

In order to write Ksp expressions properly, you must know how ionic substances dissociate in water. That means, you have to know your chemical nomenclature, polyatomic ions, and the charges associated with ion.

Also, and this is important, so pardon the shouting:

EACH CONCENTRATION IN THE Ksp EXPRESSION IS RAISED TO THE POWER OF ITS COEFFICIENT IN THE BALANCED EQUATION.

Here are some practice problems for writing Ksp expressions. Write the chemical equation showing how the substance dissociates and write the Ksp expression:

1) AlPO4
2) BaSO4
3) CdS
4) Cu3(PO4)2
5) CuSCN
6) Hg2Br2
7) AgCN
8) Zn3(AsO4)2
9) Mn(IO3)2
10) PbBr2
11) SrCO3
12) Bi2S3

Go to answers

Go to Solving Ksp Problems - Part One
Go to Solving Ksp Problems - Part Two
Go to Solving Ksp Problems - Part Three
Go to Solving Ksp Problems - Part Four

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