Multiple Solubility

All right, let's get crackin'. For those of you who forgot, a Type 2 multiple solubility problem has to do with fractional precipitation. What am I talking about? Well, when two anions form slightly soluble compounds with the same cation, or two cations form a slightly soluble compound with the same anion, the less soluble compound (the one with the smaller Ksp) will precipitate first when you add the precipitating agent (the missing cation or anion). The thing is, unless the Ksp's are about 20 orders of magnitude apart, the more soluble compound will start to precipitate before the less soluble one is finished, which can make for some ugly math (and you can bet Mr. McAfoos won't make it any easier). Generally what you have to do is find out when each thing precipitates and then see how much of the stuff you care about is left at the important point of precipitation. Fun, huh? Here are your steps:

  1. Figure out what's happening. Write down some reactions.
  2. Find all of the points of precipitation (usually two).
  3. Figure out which things precipitate in what order.
  4. Whip out your TI and plug in some numbers.

Of course, this is all pretty vague. Why don't we do a problem to clear things up?

As always, if you really feel the need, you can take a look at Type 3 problems.

Top Home