Multiple Solubility

Buckle your seat belts, kids; it's gonna be a bumpy ride. Next to polyprotic acid titrations, multiple solubility problems are the hardest problems that you're going to have to deal with on the death test.

There are basically three types of multiple solubility problems. The first type deals with dissolution by formation of a weak electrolyte, the second type deals with fractional precipitation, and the third type deals with dissolution by formation of a complex ion. For simplicity's sake, we'll call them Types 1, 2 and 3, respectively. Let's start with Type 1.

A Type 1 problem relates to dissolution by formation of a weak electrolyte. What does this mean? Well, when you want to dissolve a slightly soluble compound derived from a weak acid, you add a strong acid. When you add the strong acid, it dissociates and the hydronium effectively rips the anion off of the weak acid and forms a weak electrolyte. What this breaks down to is that basically, a Type 1 problem will ask you what amount of the acid or the hydronium ion is needed in solution to prevent precipitation of the thing to be dissolved. So how are we supposed to know how to do this?

Well, the trick to keeping the compound from precipitating is finding out the concentration of the anion at which the first trace of precipitate appears and then keeping the concentration of that ion below the point of precipitation. You will be given a Ksp for the compound to be dissolved, as well as the equilibrium constants for the acid-base reactions that are relevant to the problem.

Here are the steps you need for a Type 1 problem:
  1. Write out what reactions are going on.
  2. Find the point of precipitation for the anion.
  3. Find out what must be done to keep the concentration of the anion below the point of precipitation.

Is your head starting to hurt? Why don't you take this problem and call us in the morning?

Or, if you just can't contain your curiosity, you can go on to Type 2 problems.

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