[mookxi]

A chemist needs to adjust the pH of a 25ml calcium hydroxide solution from +10 to +8. The initial concentration of the undiluted calcium hydroxide is ..

the answer is 5 x 10^-5.

The method to work this out was:

pH=10 therefore [H+] = 10^-10
                        [OH-] = 10^-4

[Ca(OH)2] = 1/2 x 10^-4
               = 5 x 10^5

I don't understand why you times 10 ^-14 by 1/2 - where does the 1/2 come from? Is it because there is a change from +10 pH to +8? So if it was from +10 to +7 would it be 1/3? Please explain.

[Dan]

I don't understand why you times 10 ^-14 by 1/2 - where does the 1/2 come from?

You divide the 10-4 by two because for every Ca(OH)₂, you have two hydroxide ions. ie. the concentration of Ca(OH)₂ is half the concentration of OH^-

[mookxi]

Ahh, I see. Yes, that makes more sense! Thank you very much :-D

I don't understand why you times 10 ^-14 by 1/2 - where does the 1/2 come from?

You divide the 10-4 by two because for every Ca(OH)₂, you have two hydroxide ions. ie. the concentration of Ca(OH)₂ is half the concentration of OH^-