Relative Concentrations of Conjugate Acids & Bases: Acetic Acid as an Example, Study Guides, Projects, Research of Chemistry

Download Relative Concentrations of Conjugate Acids & Bases: Acetic Acid as an Example and more Study Guides, Projects, Research Chemistry in PDF only on Docsity! Relative Concentrations of Conjugate Acids & Bases Every weak acid has a conjugate weak base (and, of course, every weak base has a conjugate weak acid). For example, acetic acid, CH3COOH for example, is in equilibrium with the acetate ion, CH3COO–, as shown by the following reaction CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq) According to Le Châtelier’s principle, adding a strong acid, a source of H3O+, shifts this equilibrium to the left, increasing the concentration of CH3COOH, and adding a strong base, a source of OH–, consumes H3O+ and shifts the equilibrium to the right, increasing the concentration of CH3COO–. Clearly there are pH levels that favor acetic acid and pH levels that favor the acetate ion, although the exact pH levels are not clear from this simple treatment. The acid dissociation constant for a weak acid is a good starting point for considering how pH affects the relative amount of a weak acid and its conjugate weak base. For acetic acid the Ka expression is Ka = [H3O+][CH3COO–] [CH3COOH] Taking the log of both sides, multiplying through by –1, and substituting in pH for –log[H3O+] and pKa for –logKa gives pKa = pH − log [CH3COO–] [CH3COOH] Solving for the ratio of weak base-to-weak acid, we obtain [CH3COO–] [CH3COOH] = 10pH−pKa This equation shows us that the relative amounts of CH3COOH and of CH3COO– are determined by how close the solution’s pH is to acetic acid’s pKa value, which is 4.74. Some actual values are shown in the following table: pH pH − pKa [CH3COO–] [CH3COOH] 1.74 –3.00 0.0010 2.74 –2.00 0.010 3.74 –1.00 0.10 4.74 0 1.0 5.74 +1.00 10 6.74 +2.00 100 7.74 +3.00 1000 We can use these results to make some general statements about the relative importance of acetic acid’s conjugate weak acid and conjugate weak base forms as a function of pH. At a pH of 3.74, for example, there is one acetate ion for every 10 molecules of acetic acid; thus, for all practical purposes, at any pH < 3.74, CH3COOH accounts for more than 90% of a mass balance on acetic acid and it is the only important form of acetic acid in solution. At a pH of 5.74, there are 10 acetate ions for every one molecule of acetic acid; thus, at any pH > 5.74, CH3COO– accounts for more than 90% of a mass balance on acetic acid and it is the only