Question

Lab 5 Buffers

1. Dissolved ions in salt solutions can act as acids or bases and react with water to produce hydronium ions or hydroxide ions that contribute to the pH of the salt solution. Since strong acids and strong bases completely ionize in solution, the reverse reaction essentially does not occur, meaning that the resulting conjugate base of a strong acid or conjugate acid of a strong base do NOT act as acids or bases. Ions that are conjugate acids or bases of weak acids and weak bases do act as acids or bases.

Which ion in NaF is the conjugate base of a weak acid?

Which ion in and NH₄Cl is the conjugate acid of a weak base?

Write the ionization equilibria for the ion you named in part a acting as a base in water solution.

Write the dissociation equilibria for the ion you named in part b, acting as an acid.

2. Buffer solutions are mixtures of a weak acid and its conjugate base. When a buffer is prepared by combining 20.0 mL of 0.10 M of an unknown weak acid with 20.0 mL of 0.010 M of the conjugate base of the same acid, the observed pH was 7.55. Use this information to determine the pKa and Ka of the unknown acid. (Hint pH = pKa + log (base/acid) ; rearrange this to solve for pKa!

3. Buffer solutions work best within +/- 1 pH unit of the pKa of the weak acid present in the buffer system. Which buffer system would be the best choice to prepare a buffer solution with a pH of 4.56?

(Hint determine the pKa for the ACID in each system and establish which one has pH closest to 4.56!)

CH₃COOH/NaCH₃COO                    NH₄Cl/NH₃                 NaHCO₃/Na₂CO₃

4. How many mL of 0.20M sodium benzoate would have to be added to 25 mL of 0.10 M benzoic acid to achieve a buffer with pH = 4.00?

Answer 1

olution T)F Canion) CAnuton acid ) pH fKa tl어 ( SOlt-7 MoleoICA+1-20× o.ol_ o. L 0.2 V