How do I determine the [OH-] in a 25.0 mL of saturated solution of Calcium hydroxide.. given that 12.01 mL of 0.09666 M HCl is needed to titrate it?
First, I calculated the number of moles of OH- from Calcium hydroxide.. from that I found the total volume of solution (volume of Calcium hydroxide + volume of HCl added). In order to find the concentration of OH-, I used moles of OH- over the total volume of solution which provided me with an answer of 4.1 x 10^-5 mol/L OH-.. am I on the right track?

Also, how do I determine the solubility of Calcium hydroxide at 20.5 degrees Celsius?

Thanks in advance! :)

Hum... you need to find the concentration of OH- in the whole mixture?? I think that it was its concentration in the calcium hydroxide solution, right? If I'm wrong, then, your working is good (I didn't verify the calculations).

For the solubility at 20.5 Celsius, I use to determine such problems using a graph that I plotted and several results already given in a table or recorded. If you were not given any, then, sorry, I don't know.

Hope this helped anyway.