Question

7. Purine, a weak dibasic molecule, is present in some foods that, if eaten in excess, may cause gout in some people. Let's look at the acid/base properties of this molecule when it is protonated, H Pur pKal = 2.54 pka2 = 8.93 a. Write the NIE associated with each pK (4 pts) b. Calculate Kb for purine, Pur (this is a weak base) (3 pts) c. Calculate the initial pH of a 0.145 M purine solution. (4 pts) weak A cit 8. A solution is prepared by adding 35.00 mLs of 0.225 M purine with 45.00 mLs of 0.205 M hydrogen puring, HPur". a. Calculate the pH of this buffer solution. (7 pts) b. Calculate the pH of the solution if 10.0 mLs of 0.082 M HCl are added to the original buffer solution. Show DETAILED calculations. (10 pts) c. Could you use purine or any of its salts to make a buffer of pH 7.75? If so, which two ions or molecules would you use? (3 pts)

no net ionic is given

Answer 1

Question 2.

a. (i) Pur(aq) + H2O(l) <----> HPur⁺(q) + OH^-(aq); pKb1 = 14 - pKa1 = 14 - 2.54 = 11.46

(ii) HPur⁺ + H2O(l) <----> H2Pur⁺(q) + OH^-(aq); pKb2 = 14 - pKa2 = 14 - 8.93 = 5.07

b. Kb1 for purine = 10^-pKb1 = 10^-11.46 = 3.47*10^-12

Kb2 for purine = 10^-pKb2 = 10^-5.07 = 8.51*10^-6

c. Initial pH = 14 - {1/2 (pKb1 - Log[purine])} = 14 - {1/2 (11.46 - Log0.145)} = 7.85