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Common Ion Effect

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How valuable is the common ion effect to our understanding of solubility equilibrium?

Credit: vagawi
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[Figure1]

Let’s say you are an avid money collector. Whether it’s Indian, English, Chinese, American -- doesn’t matter, as long as it’s money! You usually organize your money into two categories: bills and coins. Since you keep decent track of your prized collection, you know that you have exactly 10,000 different coins and 1,000 different bills. You love your collection very much, but sometimes you like to have the kind of money that you can spend. This weekend, there is an Australian Expo down at the fairgrounds, and you happen to have some rare pieces -- surely you’ll take up this opportunity!

But you’re worried -- what if your collection gets significantly smaller? No worries, let’s take a look. You know that there are exactly 200 Australian coins in your collection. As for your bills collection, you only know that they make up a small portion (possibly none of them at all), so let’s refer to them as x. So added together, we have 200 + x Australian pieces. However, since x is a pretty small amount, we can assume 200 pieces in all. So, after selling our Australian pieces, our final count would be about 10,800 pieces remaining -- not bad!

This scenario is very similar to what is referred to as the common ion effect, an important concept in solubility equilibrium problems. In problems involving this concept, we assume that if a substance is found in a solution other than water, the ion of the substance that is the same as one of the ions in the solution will adopt that solution’s concentration. We also assume that the x amount that dissociates from the initial substance is negligible when added to the initial concentration of the common ion. 

Creative Applications

  1. Try this problem: The Ksp for silver carbonate is 8.4 × 10-12. The concentration of the carbonate ions in a saturated solution is 1.28 × 10-4 M. What is the concentration of the silver ions? Look up the named ions if you are unfamiliar with their formulas. Or, you may refer to the links in the "Resources" section. 
  2. Research: What are some real-world benefits of the common ion effect? You may use the links in the "Resources" section.
  3. Research: The world of chemistry is filled with special cases and exceptions. Are there any exceptions to the common ion effect? You may use the links in the "Resources" section.

Resources

http://en.wikipedia.org/wiki/Common-ion_effect

http://www.sciencegeek.net/tables/PT_ions.pdf


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