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

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