4.0 g of NaOH are dissolved per litre. Find (i) molarity of the solution (ii) `OH^(-)`ion concentration (iii) pH value of the solution (At. Masses : Na = 23, O=16, H=1).

Explanation

(i) Calculation of molarity : Mass of NaOH dissolved = 4.0 g/litre
Mol . Mass of NaOH = 40 `:.` Molarity of the solution `= ("Strength in g // litre")/("Mol . Mass") = (4.0)/(40) = 0.1 M`
(ii) Calculation of the `OH^(-)` ion conc.
NaOH completely ionizes as : `NaOH rarr Na^(+) + OH^(-) " " :. [OH^(-)]=[NaOH]=0.1 M = 10^(-1)M`
(iii) Calculation of pH :
We know that `[H_(3)O^(+)][OH^(-)]=K_(w)=1.0xx10^(-14)`
`:. [H_(3)O^(+)]=(K_(w))/([OH^(-)])=(1.0xx10^(-14))/(10^(-1))=10^(-13)M" " :. pH = - log [H_(3)O^(+)] = - log 10 ^(-13) = 13`.