C401Ch16LN3 - Chapter 16: Applications of Aqueous...

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Chapter 16: Applications of Aqueous Equilibrium Part 3 Solubilities of Ionic Compounds and K sp You’ve already learned that not all ionic compounds are water soluble. You memorized the solubility rules (page 120-121) which tell you which salt is soluble or not. But just exactly how soluble are different ionic compounds? Well, the dissolving process is an equilibrium process, so we can write an equilibrium expression for it. You can easily write the equilbrium expression for the above: The K for the dissolution or water solubility of ionic comounds is called K sp , or the solubility product constant. For calcium carbonate, K sp = 6.2x10 -12 This may seem low compared to the weak acids and bases we were just working with, but if you look at Tables of K sp values (see page A-16), you will see values as low as 10 -40 ! Practice: Write the K sp expressions for MnCrO 4 and for Pb 3 (PO 4 ) 2 K sp Problems: Finding K sp and using K sp to find Solubilities or Concentrations There are several typical problems involving K sp : Typical Problem 1) Find K sp when a salt is dissolved in pure water: A saturated solution (remember this means that as much is dissolved as is possible) of Pb 3 (PO 4 ) 2 is found to contain [Pb 2+ ] = 2.32x10 -9 M. Find K sp . (Assumption: as nothing is said, the solvent is water with no other source of lead or phosphate.) CaCO 3 ( s ) H 2 O ±² ±± ³± ±±± Ca 2+ ( aq ) + CO 3 2- ( aq )
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C401Ch16LN3 - Chapter 16: Applications of Aqueous...

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