The Common Ion Effect: Solubility

Let’s Review:

Salt: An ionic compound which consists of a cation other than H+ and anion other than OH-. A salt can be either a strong electrolyte or a weak electrolyte.

Strong electrolytes include:
• Strong acids
• Strong bases
• Most water soluble salts

Weak electrolytes include:
• Weak acids
• Weak bases
• Water insoluble salts (actually they have very, very low solubility in water and therefore are also called slightly soluble salts)
• Water

How do you know if a salt is soluble in water or not?  Memorize the rules or ….

Write and Equilibrium Constant Expression for a Slightly Soluble Salt:

The expression is called the Solubility Product Constant (Ksp)

Example: BaSO4 is a slightly soluble salt. When a reasonable quantity of solid BaSO4 is mixed with water, only a very small amount will dissolve to produce Ba+2 (aq) and SO4–2(aq)

BaSO4(s) <=> Ba+2(aq) + SO4–2(aq))

1.00 liter of saturated BaSO4 solution will contain only ~0.0025 gram of dissolved BaSO4.

The Equilibrium constant expression for this reaction can be written as:  Ksp = [BaBa+2][SO4–2]

Recall pure solids (and pure liquids) are not included in an equilibrium constant expression.

In general, MaXb(s) <=> aM+b(aq) + bX-a(aq)  is expressed as  Ksp = [M+b]a[X−a]b

These expressions are called solubility product constant expressions because they involve the product of the equilibrium concentrations of the constituent ions, each raised to the power corresponding to the number of ions in the formula.

Ksp is constant at a given temperature (van’t Hoff equation) for a saturated solution of a given compound.

Problem: Write the Ksp expression for the following weak electrolytes: Mn(OH)3(s), Sr3(AsO4)2(s), and Co2S3(s).

Calculating Solubility from Ksp:

Example: One liter of saturated calcium fluoride solution contains 0.0167 gram of CaF2 at 25oC. Calculate Ksp for CaF2.

1st: Write the balanced equation for the salt dissolving in water: CaF2(s) ó Ca+2(aq) + 2F(aq)

2nd: Calculate the moles of salt:

g CaF2 à mol CaF2

3rd: Use reaction stoichiometry to determine mol Ca+2 and mol F-:

Because there is exactly 1 L of solution, these values are also the molarities of each ion.

Ksp = [Ca2+][F-]2 = [2.14x10-4][4.28x10-4]2 = 3.9 x 10-11

Practice Problems:

1) One liter of saturated silver chloride solution contains 0.00192 g of dissolved AgCl at 25oC. Calculate Ksp for, AgCl.

2) Calculate the molar solubility of barium sulfate, BaSO4, in pure water and the concentration of barium and sulfate ions in saturated barium sulfate at 25oC.

Ksp= 1.1 x 10-10.

3) Calculate the molar solubility of strontium phosphate, Sr3(PO4)2, in pure water and the concentration of strontium and phosphate ions in saturated strontium phosphate at 25oC. Ksp= 1.0 x 10- 31.