[lgn98868]

Question (17.53 if you have Gen. Chem. Ebb&Gam 10th Edi.):

How many moles of calcium chloride can be added to 1.5 L of 0.020 M potassium sulfate before a precipitate is expected? Assume that the volume of the solution is not changed significantly by the addition of calcium chloride.

Book goes on to state the answer is 0.0018 moles.

What I know:

If anything were to precipitate it would be calcium sulfate.

The reaction for this would be CaSO₄ Ca + SO₄

This leads us to the reactant quotient [Ca²⁺][SO₄^2-]

If precipitation is going to occur, the reactant quotient (I guess it would be more proper to call it reactant product) needs to be greater than the Ksp value which is 2.4E-5.   Since we don't want precipitation (we want to get the exact Ksp value in the reaction product) we can't exceed 2.4E-5.

It is a 1:1 moles ratio and we have 0.03 moles of sulfate. Now we need to know how many moles of Ca are needed.



This is where I get stuck. I don't see how we can relate just moles to the Ksp value.

[lgn98868]

After thinking some more about this, could this be a typo in the book?
 
When dividing the Ksp value by the number of sulfate moles you get 0.0008
The book said the answer was 0.0018 moles of Calcium.

What do you think?

[Borek]

After thinking some more about this, could this be a typo in the book?

Book is right.
 

When dividing the Ksp value by the number of sulfate moles you get 0.0008

Ksp is about concentrations, not about numbers of moles.

[lgn98868]

Then is this a fractional precipitation problem? We haven't covered these in class.

[Borek]

It is a direct application of Ksp. What is the concentration of sulfate? What is the maximum possible concentration of Ca²⁺? What is the solution volume?