Struggling through Ksp

[purplepanda]

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I normally understand things pretty well, but I can't get a good grasp on these Ksp problems. I get them wrong every time!!

Here's my strategy, go to the textbooks work on a bunch..then go to Kaplan/Destroyer/ACS and I mess them all up.

Is there someone that struggled with these problems that can help me out with how you overcame the struggle? I get some right..some wrong. I don't really know what is specifically the problem.

 

[iwannabadentist]

Ksp is basically the solubility product, in short, all that means is how much of a particular salt (usually) is soluble in water (usually). The higher the Ksp, the more solute will be able to dissolve in the solvent. The basic formula for calculating Ksp is:
Ksp = [Products]/[Reactants]
So if you have something like:
A 0.2M Ca(OH)2 solution, has a Ksp of C (some small number). What will be the pH of this solution at 100% dissociation?
The way you would set this up is:
Ca(OH)2 --> Ca^2+ + 2 OH-
Now let [Ca] = x
So [OH] = 2x
Ksp = (x * (2x)^2 )/ (0.2 -3x)
but since x will be much smaller than 0.2, you can ignore this term, and you would have:
Ksp = 4x^3/0.2
Now you would solve for x, multiply it by 2 and that would give you the [OH-] at equilibrium. You would then get the pOH, and eventually the pH from there.
other than this, its a very broad concept...is there any particular question that you could post? maybe i might be able to guide you from there

 

[purplepanda]

Thank you so much for your help. 🙂 Your example helps me out a lot as I work through the problems. That's definitely one of the most specific problems, which is so helpful. I think that the broadness is what is getting to me and when I do have specific questions, they are answered in books..I just keep struggling with all the K's and pH's. I really just have to keep my head in the books.

 

[ozzie33]

Ksp is basically the solubility product, in short, all that means is how much of a particular salt (usually) is soluble in water (usually). The higher the Ksp, the more solute will be able to dissolve in the solvent. The basic formula for calculating Ksp is:
Ksp = [Products]/[Reactants]
So if you have something like:
A 0.2M Ca(OH)2 solution, has a Ksp of C (some small number). What will be the pH of this solution at 100% dissociation?
The way you would set this up is:
Ca(OH)2 --> Ca^2+ + 2 OH-
Now let [Ca] = x
So [OH] = 2x
Ksp = (x * (2x)^2 )/ (0.2 -3x)
but since x will be much smaller than 0.2, you can ignore this term, and you would have:
Ksp = 4x^3/0.2
Now you would solve for x, multiply it by 2 and that would give you the [OH-] at equilibrium. You would then get the pOH, and eventually the pH from there.
other than this, its a very broad concept...is there any particular question that you could post? maybe i might be able to guide you from there

Are you sure you didn't mean= Ksp=(X*(2x)^2/(.2-x) If you look at the stoichiometry of the reaction, for every mole of CaOH2 you get 2 moles of OH and 1 mole. The way you wrote it is 3 moles of Ca(OH)2 lost equals 1 mole of Ca and 2 moles of OH.