Calculate the pH of `10^(-8)` M acid solution.

Explanation

The molar concentration of the solution is `10^(-8)` M. Therefore, its pH value is expected to be 8. But for an acidic medium pH cannot be 8. It must be less than 7. Actually , the solution is aqueous and it has also `H^(+)` ions or `H_(3)O^(+)` ions released by the self ionisation of water. Their concentration is `10^(-7)`M.
`"Thus "" "[H_(3)O^(+)] " given by acid " =10^(-8) M`
`[H_(3)O^(+)] " given by water "=10^(-7) M`
`:. " " " Total "[H_(3)O^(+)] =10^(-8) M+10^(-7)M = 10^(-8) (1+10) =11 xx10^(-8) M`
`" Now " " "pH=- log [H_(3)O^(+)] =- log(11 xx 10^(-8)M) = (8- log 11)`
`=(8 -1.0411)= 6.9586 ~~6.96`