If the pH of a solution is 4.0 what is the concentration of oh in the solution?

To find the concentration of hydroxide ions ([OH-]) in a solution when the pH is 4.0, you can use the formula pH + pOH = 14. Since the pH is 4.0, the pOH would be 14 - 4 = 10. To convert pOH to [OH-] concentration, use the formula [OH-] = 10^(-pOH). Thus, [OH-] = 10^(-10) = 1 x 10^(-10) M.