There are two ways to solve this problem. I will show both ways. First, you can find the pOH. The formula for pOH is: pOH = -log([OH-]). You take the negative log of the concentration of OH-. pOH = -log(0.15M) = 0.824 Now, pH + pOH = 14. So, you can subtract both sides by pOH to get pH. 14 - pOH = 14 - 0.824 = 13.176 = pH The second way to solve this problem is to find the concentration of H+ ions. We know that [H+] x [OH-] = 1 x 10^-14. So, you can solve for [H+] [H+] = (1 x 10^-14)/(0.15) = 6.67 x 10 ^ -14 Now, you can take - log([H+]) to get pH. pH = -log(6.67x10^-14) = 13.176. Notice you get the same answer. Also, notice when you take -log([OH-]) you get pOH. When you take -log([H+]) you get pH. Hope this helps and please rate. :) Solution There are two ways to solve this problem. I will show both ways. First, you can find the pOH. The formula for pOH is: pOH = -log([OH-]). You take the negative log of the concentration of OH-. pOH = -log(0.15M) = 0.824 Now, pH + pOH = 14. So, you can subtract both sides by pOH to get pH. 14 - pOH = 14 - 0.824 = 13.176 = pH The second way to solve this problem is to find the concentration of H+ ions. We know that [H+] x [OH-] = 1 x 10^-14. So, you can solve for [H+] [H+] = (1 x 10^-14)/(0.15) = 6.67 x 10 ^ -14 Now, you can take - log([H+]) to get pH. pH = -log(6.67x10^-14) = 13.176. Notice you get the same answer. Also, notice when you take -log([OH-]) you get pOH. When you take -log([H+]) you get pH. Hope this helps and please rate. :)..