Specify the reagents (an acid and its conjugate base or a base and its conjugate

Question

Specify the reagents (an acid and its conjugate base or a base and its conjugate acid) and the concentration of each reagent needed to prepare buffer solutions having the listed pH values. NOTE: The optimum buffer solution is one with equal concentrations of the weak acid (weak base) and its conjugate base (conjugate acid). Under these conditions, the pH of the solution is equal to the pKa (pKb). So the best reagent for each of the solutions below is one whose pK is equal to the pH. Since the tables in the appendix list K values, each of the pH's must be converted to their corresponding [H+] and compared to an equilibrium constant in Appendix D.

a) 4.74

b) 9.81

c) 2.92

Explanation / Answer

a) 4.74
pH = 4.74 : [H+] = 1.8 x 10–5 Acetic acid has a Ka = 1.8 x 10–5;
therefore, a solution of .85 M HC2H3O2 and .85 M NaC2H3O2 would
have a pH of 4.74.

b) 9.81
pH = 9.81 : [H+] = 1.54 x 10–10 M and [OH-] = 6.4 x 10–5 M. Since the
pH is basic, a weak base and its conjugate acid should be considered.
Ethylamine has a Kb = 6.4 x 10–5; therefore, a solution of 0.70 M
(CH3)2N and . M (CH3)2NHCl would have a pH of 9.81.

c) 2.92
pH = 2.92 : [H+] = 1.2 x 10–3 Chloroacetic acid has a Ka = 1.4 x 10–3;
therefore, a solution of .65 M HC2H2O2Cl and .76 M NaC2H2O2Cl
would have a pH of 4.74.

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