Biochemistry Quiz

Biochemistry Quiz

Assume an activity coefficient of 1 for all substances and no effect of ionic strength. Eliminate terms in quadratic solutions for [H+] only if the weak acid is dissociated <5%. Use the pKa values given with the problems.

Phosphoric acid (H3PO4) is a triprotic acid

pKa1 = 2.12           H₃PO₄ <—> H⁺ + H₂PO₄^–

pKa2 = 7.21           H₂PO₄^2- <—> H⁺ + HPO₄^2-

pKa3 = 12.32        HPO₄^2- <—> H⁺ + PO₄^3-

1.
(a) At what pH is [HPO42-] double that of [ PO43-]? (5 pts)
(b) At what pH is [HPO42-] equal to [ PO43-]? (5 pts)
(c) At what pH is [HPO42-] half that of [ PO43-]? (5 pts)
(d) Guided by your answers in parts (a) through (c), explain the relationship between [acid], [conjugate base] and pKa.
(5 pts)

2.
(a) If 6 volumes of 12 mM KOH are mixed with 1 volume of 45 mM H3PO4, what will be the pH of the final mixture?
(10 pts)
(b) If 30 μmoles of lactic acid is generated in a 4.0 ml enzymatic reaction buffered by 50 mM Na- phosphate (pH 7.0), what will be the final pH of the reaction mixture? Lactic acid pKa = 3.86
(10 pts)