# Chemistry: Ksp calculation confusion

1. Nov 21, 2013

### UselessLadder

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

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

3. 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?

2. Nov 21, 2013

### Staff: Mentor

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.

3. Dec 5, 2013

### 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. :)

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