Chemistry: Ksp calculation confusion

[UselessLadder]

Homework Statement

I have to calculate the Ksp value for calcium hydroxide from experimental data. We titrated 10.00 mL of saturated Ca(OH)2 aqueous solution with 11.00 mL of 0.01607 M EDTA.

Homework Equations

The Ksp formula is given in my manual as [Ca^2+]*[OH^-]^2

The Attempt at a Solution

I got the # of moles of EDTA by multiplying molarity by liters = 0.0001768 mol.

Since the stoichiometric ratio of Ca^2+ to EDTA is 1:1, 0.0001768 mol of ca^2+ have been titrated.

Now I divided that by 0.010 L to get the [Ca^2+] = 0.01768 M

I got the OH^- concentration by multiplying 0.01768 x 2 = 0.03535 M.

Using the above formula, [001768]*[0.3535]^2 = 2.209*10^-5

Herein is the problem: my book gives the Ksp as 8.0*10^-6. So my result is pretty far off, giving me a percent error of 176%. Am I doing something wrong? Did I lose a zero somewhere or is my experimental data flawed?

 

[Borek]

Your calculations look OK to me. Doesn't mean your titration was right - unless I am mistaken it gives solubility of 0.13 g per 100 mL of the solution, while it should be closer to 0.17 g/100 mL at the room temperature.

Determining total concentration of calcium in the solution is not a good method of Ksp determination. Substantial part of Ca²⁺ is in the form of a CaOH^- complex - while this calcium will get complexed by EDTA, it is not directly involved in Ksp. That's not something you can do anything about using the method you are using.

 

[UselessLadder]

(Sorry for the super late reply)
You're right, that's a terrible way of getting Ksp. All my classmates got equally far-off results, but there wasn't anything to be done about it the way the lab was designed. Still, I got an A, so go figure. :)