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 ?

Options

A. `5.6 xxx 10^(-5)`
B. `6.5 xx 10^(-5)`
C. `4.5 xx 10^(-5)`
D. `5.4 xx 10^(-5)`

Answer

A

Explanation

We know that pH + pOH = 14
pOH = `(14 -10.65) = 3.35`
`[OH^(-)] = 10^(-3.35) = 4.47 xx 10^(-4) M `
One molecule of `Ca(OH)_(2)` furnishes `2OH^(-)` ions .
Hence concentration of `Ca(OH)_(2) = (4.47 xx 10^(-4))/2 `
`= 2.235 xx 10^(-4)` M
No of moles in 250 mL = `(2.235 xx 10^(-4))/4 = 5.6 xx 10^(-5)`