Multiple Solubility


Moving on, we come to our final type of multiple solubility problem, the Type 3 problem. Type 3 problems are by far the most complex of the multiple solubility problems. You will have one of these on the Death Test. This problem covers dissolution by formation of a complex ion. Here's the deal: a lot of slightly soluble electrolytes dissolve when you add an acid or base that will form a complex ion (such as Ag(S2O3)23-, as you'll soon see). What this means to us is that a Type 3 problem will ask you how much of the acid or base you have to put in solution to dissolve the electrolyte. You will be given the Ksp for the electrolyte and the formation constant (Kf) for the complex ion.


The key to solving this type of problem is pretty much the same as for Type 1, except that this time you're dealing with a formation reaction instead of an acid-base reaction, so you have to take into account the stuff left over after the formation. Here are your steps (we will call the electrolyte AB and the complex ion A(CD)):


  1. Write out reactions (dissociation reaction and formation of complex ion).
  2. Find out which ion you care about in the electrolyte (A in AB).
  3. Find out how much of the other ion (B) is in solution.
  4. Find the point of dissolution (same as point of precipitation) of the electrolyte.
  5. Find the amount of the other part of the complex ion (CD in A(CD)) that will remain in solution after equilibrium has been established.
  6. Add that to the amount of CD that is used in the formation of the complex ion (A(CD)).


Clear as New Jersey's drinking water, eh? Well, I'm sure we can make it all better by doing a problem.


Or, if you've got ants in your pants, skip on ahead to the exciting Conclusion.


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