Question

a OH Old Exam Question Example #2 Imagine you have a 0.125 M aqueous solution of aspirin, an acid drug with pk, = 3.5, in equilibrium. a) Estimate the pH of the solution Hint: Determine if it is an acid or base solution, weak or strong If weak acid/base, first try (H*] = KC, or [OH-] = K,C, b) Predict what would happen to the pH when you add more H20 Hint: First figure out what happens to the concentrations HA(aq) + H2O(1) ZA" (aq) + H20*(aq) Use both kinetic and thermodynamic arguments.

Answer 1

a) we have the relation: K_a = 10^-pKa = 10^-3.5 = 3.16 * 10^-4

Since it is a weak acid, [H⁺] = (K_a * C)^0.5 = (3.16 * 10^-4 * 0.125)^0.5 = 6.28 * 10^-3 M

pH = -log([H⁺]) = -log(6.28 * 10^-3 ) = 2.20

b) as we add more water molecules, the concentration of acid decreases meaning that the concentration of H⁺ will decrease amd hence the pH increases