gchem destroyer problem

[dentalstudent2021]

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The Ksp of PbCl2 is 1.6E-5 at 25 degrees C.

What is the solubility of PbCl2 in.01M KCl?

-- now I get how to do Ksp problems, atleast I think, But the part I don't get in this prob is why 2x in
ksp = (x)(2x+0.1)^2 is ignored.

Also could someone tell me the name of this specific "rule" so I can read up on it. thanks!

 

[cacajuate]

It's because the .1 is substantially larger than the 2x so you just leave it out the 2x.


Like Chad says, you take one dollar from bill Gates bank account, he ain't going to notice. So leaving the 2x out doesn't affect anything because it is so small.

 

[dentalstudent2021]

thanks for the reply.

It's because the .1 is substantially larger than the 2x so you just leave it out the 2x.

--so how did you know that 0.1 is bigger than 2x ? we have'nt solved for x yet. whats the reasoning.

 

[cacajuate]

Honestly, I don't know that answer. I just know that you always drop the x value everytime you do a common ion problem.

 

[DAT Destroyer]

It's because the .1 is substantially larger than the 2x so you just leave it out the 2x.


Like Chad says, you take one dollar from bill Gates bank account, he ain't going to notice. So leaving the 2x out doesn't affect anything because it is so small.

The 2x term comes from the fact that 2 moles of Cl- is produced from 1 mole of the compound. This must be taken into account. See any standard GenChem text on the common ion effect.

Hope this helps.

Dr. Romano

 

[w4nt2baDDS]

The Ksp of PbCl2 is 1.6E-5 at 25 degrees C.

You know the x will be substantially smaller because of the Ksp value given

Ksp=[Pb][Cl]^2

Now, if you claim the concentration of Pb to be X then the relative concentration for Cl would be 2X.

This gives us: 1.6x10^-5 = [X][2X]^2 ========> 1.6x10^-5 = 4X^2 (considering how X squared times 4 equals .000016 you can see X is going to considerably smaller than .1)

For thoroughness, lets see how much a difference the X would make if they were considered in the equation:

4x10^-6 = X^2 and thus X = 2x10^-3 thus X = .002

If you consider X's contribution: (.104)^2 = .0108

Without X's contribution: (.1)^2 = 0.0100

If you consider the difference: 100% - (.0100/0.0108)*100%= 7.4%

That's actually not the worlds smallest difference lol

BUT, for the DAT that typically can be ignored.

Main message, the smaller the Ksp the smaller the X values typically.. which means the more likely you ignore them if a common ion is present

 

[dentalstudent2021]

w4nt2baDDS - Thank a lot mate