Common Ion Effect

[dmplz707]

Hey guys so in terms of calculation problems for the common ion effect, would I be safe for the MCAT just drilling in my head that the x terms can simply be ignored all the time? For example;

What is the molar solubility of CaCl2 (s) in 0.01 M NaCl (aq) solution?

CaCl2(s) <-> Ca2+ (aq) + 2Cl- (aq) Ksp = 2.5x10^-10 M^3

Doing all the math...I have it down to...

Ksp = [Ca2+][Cl-]^2 = (x)(0.01 + 2x)^2

So would I be safe if I just ignored the 2x term? Thanks guys!

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[Majik]

Yeah, for sure. Otherwise you'd have to use the quadratic equation and on a timed test, that's highly unlikely. Usually though, for common ion effect questions its mostly conceptual. They'll ask whether molar solubility is increased or decreased in the presence of a common ion, and of course the answer would be that it decreased. They also could throw in a different ion (complex ion effect) that reacts with one or both of the ions to form some other salt (this is where knowing solubility rules is helpful, although I admit this stuff is most likely explained in a passage. Don't waste your time learning SR unless you have absolutely nothing else to do). This would increase the solubility of the salt. An example is Mg(OH)2 that is dissolved in an acidic solution. Because OH- reacts with H+, more Mg(OH)2 dissolves, so the reaction proceeds in the forward direction.

 

[iamtony]

sorry for thread jacking but it appeared your original question had been answered and my question pertains to the same thing.

theoretically,if by any chance say the calcium had a coefficient of 2 then it would be 2 Ca
would i continue on and the resulting equation would contain (2x)^2 = 4x^2 ?

and therefore to solve for ksp it would be: ksp = (4x^2) * (.01)^2

probably won't show up though right?

 

[ktran17]

I would to add that when it comes to most solubility questions, just refer back to Le' Chatlier's principle.

 

[Temperature101]

The 2x should be tossed out...

On a side note: Is it "The common ion effect" or "The common effect ion"? Who cares...English is my third language anyway...

 

[iamtony]

The 2x should be tossed out...

On a side note: Is it "The common ion effect" or "The common effect ion"? Who cares...English is my third language anyway...

sorry are you referring to me or the op?

and never heard it called common effect ion before...