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7.6 Solubility Equilibrium and Ksp
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While some ionic compounds dissolve entirely in water, some are only sparingly soluble
Equilibrium exists between the solid ionic compound and its dissociated ions in solution
As we know, the concentration of a solid is constant; therefore we can introduce a new
equilibrium constant… the solubility product constant
CA(s)  C+(aq) + A-(aq)
Ex)
Ksp = [C+(aq)] [A-(aq)]
Write the Ksp for copper(II) phosphate
Calculating Ksp Using Molar Solubility
Ex)
The solubility of Ag2CO3 is 1.3x10-4 mol/L @ 25°C. Calculate Ksp for silver carbonate.
Calculating Molar Solubility from Ksp
Ex)
Calculate the molar solubility of Cu(OH)2 (s) in water.
(Ksp is 1.6x10-19)
The Common Ion Effect
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What happens if you add a solid to a solution that already contains one or more of its
ions?
Look at:
PbI2 (s)  Pb+2 (aq)
+
2I- (aq)
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If the solution already contains I- such as NaI(aq) then fewer I- ions can form from PbI2 (s)
and thus the equilibrium shifts to the left – more precipitate (less soluble)
a)
At 25C, Ksp for PbI2 is 9.8 x 10-9. What is the molar solubility of PbI2 in water at 25C?
b)
Determine the solubility of PbI2 (s) in a 0.10 M solution of NaI.
Self Check
How prepared am I to start my homework? Can I …
… write a Ksp equation
… calculate the value of Ksp
… calculate the Molar Solubility of a ionic compound
… calculate molar solubility with a common ion present
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