For the equilibria between an ionic solid and its dissolved ions, the equilibrium constant is called the solubility product Ksp.

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Ksp gives us information about the solubility of the compound. The lower Ksp is, the less soluble a compound is.

Eg: CuCl2 Ksp=1.7×10-7 at 25°C        NaCl Ksp= 3.8×101 at 25°C      CuS Ksp=6.0×10-37 at 25°C

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Solubility: The concentration of a dissolved solid in g/100mL. If stated in M is called molar solubility.

Calculating Ksp from solubility

Calculate solubility using Ksp

Predicting Precipitation:

The reaction quotient can tell us whether a compound will form a precipitate or not by comparing it to Ksp.

  • If Q = K the solution is saturated and there should be no precipitate formed
  • If Q < K, the solution is unsaturated and there should be no precipitate formed
  • If Q > K, the solution is supersaturated and the equilibrium will shift to the left to reduce the amount of products, creating a precipitate

Ex: if 100 mL of 0.100 M calcium chloride are mixed with 100 mL of 0.0400 M sodium sulfate at 20°C, will a precipitate form? Hint, the Ksp of calcium sulfate is 3.6×10-5 at 20°C.

Since Q > K, a precipitate will form.

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