Assume that acid rain has lowered the ph of a particular lake to ph 4.0. what is the hydroxyl ion concentration of this lake?

            [OH⁻]  =  1.0 × 10⁻¹⁰ M

             In order to calculate concentration of hydroxide ion i.e. [OH⁻] first we will find the pOH by using following relation,

                                       pH  +  pOH  =  14

Putting value of pH given and solving for pOH,

                                       4.0  +  pOH  =  14

                                       pOH  =  14  -  4.0

                                       pOH  =  10.0

As ,

                                       pOH  =  -log [OH⁻]

So,

                                       [OH⁻]  =  10^-pOH

Putting value of pOH,

                                       [OH⁻]  =  10⁻¹⁰

                                       [OH⁻]  =  1.0 × 10⁻¹⁰ M

since electrons are negative the answer is neagtaive