Correct Answer - 13
Mass of NaOH =4.0 g
Molar Mass of NaOH =40.0 g `" mol"^(-1)`
No. of moles of NaOH `=((4.0g))/((40.0g " mol"^(-1)))=01" mol "`
Volume of the solution =1.0 L
`:.` Molar concentration of solution `=((0.1 "mol "))/((1.0L)) =0.1 " mol " L^(-1) =0.1 M`
As NaOH is a strong base it is completely ionised in water as :
`NaOH overset((aq))(to) Na^(+) (aq) + underset(0.1M)(OH^(-) (aq))`
`:. "" "[OH^(-)] =0.1 M`
`[H_(3)O^(+)]=(K_(w))/[[OH^(-)]] =((1.0 xx 10^(-14) M^(2)))/(0.1M) =10^(-13)M`
`pH =- log [H_(3)O^(+)] =- log (10^(-13)) -(-13 log 10) =13`
