common ion effect

[dextor2003]

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90. The addition of a strong base to a saturated solution of Ca(OH), would:
A. decrease the number of OH- ions in solution.
B. increase the number of Ca2 + ions in solution.
C. cause Ca(OH), to precipitate.
D. decrease the pH.

i was stuck b/w a and c on this one, but hte answer turned out to be c. i thought the hydroxide concentration would also decrease since some molecules would go into forming the precipitate...but then i realized that you're also initially adding hydroxide...so would the OH concentration just stay the same ?? ...or increase ??

 

[golgiapparatus88]

It's Le Chatelier's Principle. If you write out Ca(OH)2 -> Ca2+ + 2OH and increase the [OH] concentration, the reaction will shift towards the reactants side (left).

 

[collegestud2013]

use Ksp = [Ca2+][OH-]^2 and then apply Le Chatelier stuff

 

[loveoforganic]

You're adding a strong base (and thus tons of OH-). [OH-] will increase - it will just be from a different source than the calcium hydroxide.