What is the hydroxide ion concentration of the lake described in question 3?

The hydroxide concentration is written as [OH-]. A useful equation is that pH + pOH = 14. Since the pH is 4, the pOH must be 10. To get [OH-], again take the neg. anti-log of both sides: - anti log (pOH-) = - antilog (10) - - - > [OH-]=1 x 10^-10 M or 1 x 10^-10 moles [OH-] per liter solution (in this case, lake).

The hydroxide ion concentration describe in question 3 is 10-10 M