The pH of a solution is way of expressing the concentration of hydrogen ions, H+. In any solution there will be both H+ and OH- ions. If the hydrogen ion concentration, [H+] is greater than the [OH-] concentration the solution is acidic, and of course, vice versa. Start by determining the molarity of the OH- ions. 0.16g NaOH x (1 mol NaOH / 40.0 g NaOH) = 0.0040 mol NaOH and 0.0040 mol OH- Molarity = 0.0040 mol / 2 L = 0.0020 M OH- pOH = -log[OH-] = -log(0.0020) = 2.70 pH + pOH = 14.00 at 25C pH = 14 - pOH = 11.30 ======== Follow up ========= HPV has made an error that, while simple, will impact the pH a great deal. He has written the hydroxide ion concentration as 0.0002, rather than 0.002. This shift of 1 order of magnitude will make a 1 unit difference in the pH.