In order to find the OH- concentration of a solution, we need to know the pOH of the solution, but in this question, we are provided with the pH, which is equal to 4.0, but the sum of pH and pOH is always 14, therefore if we use this logic, we can find pOH:
pH + pOH = 14
pOH = 14 - 4
pOH = 10
Now that we have the pOH, we can find the OH- concentration, using the following formula:
[OH-] = 10^-pOH
[OH-] = 10^-10
[OH-] = 1*10^-10
