Stoichiometric calculations buffered solutions

When a strong acid or base is added to a buffered solution, it is best to deal with the stoichiometry of the resulting reaction first. After the stoichiometric calculations are completed, then consider the equilibrium calculations. This procedure can be represented as follows ... 

To calculate how the pH of a buffer solution changes when small amounts of a strong acid or base are added, we must first use stoichiometric principles to establish how much of one buffer component is consumed and how much of the other component is produced. Then the new concentrations of weak acid (or weak base) and its salt can be used to calculate the pH of the buffer solution. Essentially, this problem is solved in two steps. First, we assume that the neutralization reaction proceeds to completion and determine new stoichiometric concentrations. Then these new stoichiometric concentrations are substituted into the equilibrium constant expression and the expression is solved for [H30 ], which is converted to pH. This method is applied in Example 17-6 and illustrated in Figure 17-6. 

In parts (a) and (b) we complete essentially the same calculations. We should recognize that we are adding a strong acid or a strong base to the buffer solution. To investigate this effect we make a stoichiometric calculation, followed by an equilibrium calculation. The stoichiometric calculation is necessary to account for the neutralization of the base or acid components of the buffer. 

Suppose we are titrating the triprotic acid H P04 with a solution of NaOH. The experimentally determined pH curve is shown in Fig. 11.13. Notice that there are three stoichiometric points (B, D, and F) and three buffer regions (A, C, and E). In pH calculations for these systems, we assume that, as we add the hydroxide solution, initially NaOH reacts completely with the acid to form the diprotic conjugate base... 

When plotted on a graph of pH vs. volume of NaOH solution, these six points reveal the gross features of the titration curve. Adding additional calculated points helps define the pH curve. On the curve shown here, the red points A-D were calculated using the buffer equation with base/acid ratios of 1/3 and 3/1. Point E was generated from excess hydroxide ion concentration, 2.00 mL beyond the second stoichiometric point. You should verify these additional five calculations. 

Before the stoichiometric point is reached, the solution is a buffer with differing amounts of acid/conjugate base. To calculate the pH, follow the procedure outlined in Example 11.6 in the text. At the stoichiometric point, the analyte solution consists of the salt of a weak acid or of a weak base. To calculate the pH, fol low the procedure outlined in Example 11.5 in the text. 

Before the stoichiometric point is reached, the solution is a buffer with differing amounts of acid/conjugate base. To calculate the pH, follow the procedure outlined in Example 13.6 in the text. 

NaOH converts a stoichiometric amount of the acid into its conjugate base. As we calculated previously, 5.00 mL of the 0.100 M NaOH solution contains 0.000500 mol OH . When the 0.000500 mol OH is added to the weak acid solution, the OH reacts stoichio-metrically with HCHO2 causing the amount of HCHO2 to decrease by 0.000500 mol and the amount of CHO2 to increase by 0.000500 mol. This is very similar to what happens when you add strong base to a buffer, and it is summarized in the following table ... 

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