Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry231cies is the limiting reagent. The following example shows how we can usethe ladder diagram in Figure 6.5 to evaluate the result of mixing togethersolutions of acetic acid and p-nitrophenolate.Example 6.8Predict the approximate pH and the final composition of mixing together0.090 moles of acetic acid and 0.040 moles of p-nitrophenolate.So l u t i o nThe ladder diagram in Figure 6.5 indicates that the reaction between aceticacid and p-nitrophenolate is favorable. Because acetic acid is in excess, weassume that the reaction of p-nitrophenolate to p-nitrophenol is complete.At equilibrium essentially no p-nitrophenolate remains and there are 0.040mol of p-nitrophenol. Converting p-nitrophenolate to p-nitrophenol consumes0.040 moles of acetic acid; thusmoles CH 3 COOH = 0.090 – 0.040 = 0.050 molmoles CH 3 COO – = 0.040 molAccording to the ladder diagram, the pH is 4.76 when there are equalamounts of CH 3 COOH and CH 3 COO – . Because we have slightly moreCH 3 COOH than CH 3 COO – , the pH is slightly less than 4.76.Practice Exercise 6.6Using Figure 6.5, predict the approximate pH and the composition of asolution formed by mixing together 0.090 moles of p-nitrophenolate and0.040 moles of acetic acid.Click here to review your answer to this exercise.If the areas of predominance for an acid and a base overlap, then practicallyno reaction occurs. For example, if we mix together solutions ofCH 3 COO – and p-nitrophenol, there is no significant change in the molesof either reagent. Furthermore, the pH of the mixture must be between4.76 and 7.15, with the exact pH depending upon the relative amounts ofCH 3 COO – and p-nitrophenol.We also can use an acid–base ladder diagram to evaluate the effect ofpH on other equilibria. For example, the solubility of CaF 2CaF Ca F2 () s 2 + ( aq ) −+ 2 ( aq )is affected by pH because F – is a weak base. Using Le Châtelier’s principle,converting F – to HF increases the solubility of CaF 2 . To minimize thesolubility of CaF 2 we need to maintain the solution’s pH so that F – is thepredominate species. The ladder diagram for HF (Figure 6.6) shows us thatmaintaining a pH of more than 3.17 minimizes solubility losses.more basicF –pH pK a= 3.17HFmore acidicFigure 6.6 Acid–base ladder diagramfor HF. To minimize the solubilityof CaF 2 , we need to keepthe pH above 3.17, with morebasic pH levels leading to smallersolubility losses. See Chapter 8 fora more detailed discussion.