Water Dissociation Water acts as both acid and base Dissociates into H3O+ and OH- ions 2H2O(l) H3O+(aq) + OH-(aq) Rewritten, H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Water Dissociation (cont.) Rate of reaction reaches equilibrium and allows [H3O+] and [OH-] to be determined 2H2O(l) H3O+(aq) + OH-(aq) Kw = [H3O+] [OH-] Kw = water dissociation constant = 1x10-14for pure water [H3O+] = 1x10-7 M in pure water [OH-] = 1x10-7 M in pure water Acid/Base Addition Kw = [H3O+] [OH-] = 1 x 10-14 regardless of where ions come More acid, increase [H3O+] /decrease [OH-] 2 sources for [H3O+] , so less water dissociation More base, decrease [H3O+] /increase [OH-] pH Measure of the concentration of [H3O+] ions in an acidic/basic solution Uses logarithmic scale pH = -log[H+] or –log[H3O+] [H3O+] = 10-pH pOH = -log[OH-] pH + pOH = 14 pH Scale Logarithmic scale. Measures the concentration of hydrogen ions [H+] in a solution. Range from 0-14. NEUTRAL, pH=7. (pure water) BASE, pH > 7. (ocean water, milk of magnesia, baking sodea) ACID, pH < 7. (stomach acid/HCl, vinegar, soft drinks) pH scale (cont.) Increase [H3O+], decrease pH value, decrease [OH-] Decrease [H3O+], increase pH value, increase [OH-] pH Scale How do we measure the pH of a solution? Acid-base indicators Weak acids/bases (ex. litmus paper) pH meter Equations pH = -log[H+] pOH = -log[OH-] pH + pOH = 14 [H+][OH-] = 1x10 -14 M2 Example 1 What is the pH of Pepsi Cola if the [H3O+] in the solution is 0.0035M? pH and strong acids Strong acids completely dissociate in solution SOOO solution’s [H3O+] ~ [H3O+] in acid All [H3O+] ions resulting from acid Example 2 Find the pH of a 2.5M HCl solution. Example 3 Find the pH of a 0.05M H2SO4 solution + O] Why is the [H3 from water dissociation not a factor? 1) [H3O+] = 1x10-7 M in water not a big number 2) LeChatlier’s Principle As more H3O+ ions dissociate from water, reaction shifts to LEFT to compensate If more H3O+ ions added to solution, water reforms to try and compensate SOOO decrease in [H3O+] from water dissociation Homework Acid/Base Introduction Worksheet pH calculation Problems