How many moles of `Ca(OH)_(2)` must be dissolved to produce 250 mL of an aqueous solution of pH `10.65`, assuming completer dissociation ?

Explanation

Let a mole of `Ca(OH)_2` be dissolved in 250 mL solution to have pH =10.65
`[Ca(OH)_(2)]=(a xx 100)/(250)=4aM`
`Ca(OH)_(2) to Ca^(2+)+2OH^(-)` Complete ionization
Initial moles `4a" 0 0"`
Final moles `0" 4a "2 xx 4aM`
`therefore [OH^(-)]=8aM`
`[H^(+)]=(10^(-14))/(8a)`
`(10^(-14))/(8a)=2.238 xx 10^(-11)`
`therefore a=5.58 xx 10^(-5)"moles"`