Assessing buffer action

Estimate the effect of addition of 0.020 mol of hydronium ions (from a solution of a strong acid, such as hydrochloric acid) on the pH of 1.0 dm of (a) 0.15 M CH3COOH(aq) and (b) a buffer solution containing 0.15 m CH3COOH(aq) and 0.15 m NaCH3C02(aq). 

Strategy Before addition of hydronium ions, the pH of solutions (a) and (b) is 2.8 Example 4.5) and 4.8 (see the preceding brief illustration). After addition to solution (a) the initial molar concentration of CH3COOH(aq) is 0.15 m and that of H30+(aq) is (0.020 mol)/(1.0 dm ) = 0.020 m. After addition to solution (b), the initial molar concentrations of CH3COOH(aq), CH3C02(aq), and H30+(aq) are 0.15 m, 0.15 m, and 0.020 m, respectively. The weak base already present in solution, CH3C02(aq), reacts immediately with the added hydronium ions  

Solution For addition of a strong acid to solution (a), we draw up the following equilibrium table to show the effect of the addition of hydronium ions  

The value of x is found by inserting the equilibrium concentrations into the expression for the acidity constant  

As in Example 4.5, we assume that x is very small in this case x . 0.020, and write 

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H CO, can therefore be replaced in the mass action equation by PCO,. At this point, an understanding of the role of the bicarbonate buffer system in assessing clinical acid-base disorders can be achieved simply by reference to the relation.ship ... 

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