Analytical Chem istry - DePauw University

230 Analytical Chemistry 2.0more basicO 2NO –Example 6.7Draw a ladder diagram for the weak base p-nitrophenolate and identify itspredominate form at a pH of 6.00.pH pK a= 7.15O 2Nmore acidicOHSo l u t i o nFigure 6.4 Acid–base ladder diagramfor p-nitrophenolate.Practice Exercise 6.5To draw a ladder diagram for a weak base, we simply draw the ladder diagramfor its conjugate weak acid. From Appendix 12, the pK a for p-nitrophenolis 7.15. The resulting ladder diagram is shown in Figure 6.4. At apH of 6.00, p-nitrophenolate is present primarily in its weak acid form.Draw a ladder diagram for carbonic acid, H 2 CO 3 . Because H 2 CO 3 isa diprotic weak acid, your ladder diagram will have two steps. What isthe predominate form of carbonic acid when the pH is 7.00? Relevantequilibrium constants are in Appendix 11.Click here to review your answer to this exercise.Ladder diagrams are particularly useful for evaluating the reactivity betweena weak acid and a weak base. Figure 6.5 shows a single ladder diagramfor acetic acid/acetate and p-nitrophenol/p-nitrophenolate. An acid and abase can not co-exist if their respective areas of predominance do not overlap.If we mix together solutions of acetic acid and sodium p-nitrophenolate,the reaction6.32occurs because the areas of predominance for acetic acid and p-nitrophenolatedo not overlap. The solution’s final composition depends on which spe-O 2NO –pHpK a= 7.15CH 3COO −O 2NOHpK a= 4.74CH 3COOHFigure 6.5 Acid–base ladder diagram showing the areas of predominance for acetic acid/acetate and for p-nitrophenol/p-nitrophenolate. The areas in blue shading show the pH range where the weak bases are the predominate species;the weak acid forms are the predominate species in the areas shown in pink shading.