[AussieKenDoll]

I performed a lab experiment to Determine the solubility product of Calcium Hydroxide and after pouring the solid free supernatant to a beaker, I pipetted out 25 mL of supernatant solution and run that volume into a 25 mL standard HCl (0.1M) solution and mixed well
Then I titrated the total volume of the solution with standard NaOH solution(0.1M) using Phenolphthalein indicator according to the Lab experiment guide book.

Why we add HCl to the supernatant solution? Why we can't titarate the supernatant solution directly with NaOH?
or why we titrate Saturated Calcium Hydroxide solution directly NaOH,Rather than with HCl?

[chenbeier]

Why we add HCl to the supernatant solution? Why we can't titarate the supernatant solution directly with NaOH?
or why we titrate Saturated Calcium Hydroxide solution directly NaOH,Rather than with HCl?

What is the pH of the  supernatant solution?  Think what you realy doing with this method.. NaOH can only react with acid not with base.

[AussieKenDoll]

How could I find the Ksp of the Calcium Hydroxide from the above details?

[Borek]

Hint: you have enough information to determine concentration of dissolved Ca(OH)₂.

[AussieKenDoll]

Yes, I have enough information to determine the concentration of dissolved Ca(OH)2 But why when determining the Solubility product of Calcium Hydroxide we were asked to pipette out 25 mL of Filtered Calcium Hydroxide supernatant solution and run that volume to a 25 mL of Standard HCL of 0.1M and then titrate the total volume of  solution with standard NaOH solution of 0.1 M rather than  titrating  saturated Calcium Hydroxide supernatant solution directly with HCl.
Why we had to do that?

I don't understand what is the purpose of mixing HCl with the saturated Ca(OH)₂ supernatant solution before titrating it with NaOH?

[chenbeier]

It is a back titration.  You mix the supernatant with HCl and neutralize it. With the NaOH you titrate the HCl what is left. The difference will guide you to the Calcium hydroxide amount. Theretically you could also titrate directly the Calciumhydroxide, but it is a weaker base, so the change of indicator is not good visible. For this reason the back titration was used.

[AussieKenDoll]

@chenbeier OMG thank you so much! It helped me a lot! I wrote my lab report! Thanks!