17.1 Student Notes

IV. Acid-Base Equilibria

A. Acid-Ionization Equilibria

1. Acid ionization – acid dissociation –

2. Acid ionization constant – acid-dissociation constant –

3. Solving for acid ionization

HA (aq) + H₂O (l) ¯ H₃O⁺ (aq) + A^- (aq)

4. Experimental Determination of K_a

a. Method 1 – Electrical conductivity

Degree of ionization –

Percentage –

b. Method 2 – From pH

5. Determining K_a from the Solution pH

Nicotinic acid (niacin) is a monoprotic acid with the formula HC₆H₄NO₂. A solution that is 0.012 M in nicotinic acid has a pH of 3.39 at 25° C? What is the degree of ionization of nicotinic acid in this solution?

6. Calculations with K_a

Calculating Concentrations of Species in a Weak Acid Solution Using K_a (Approximation method)

What are the concentrations of nicotinic acid, hydrogen ion, and nicotinate ion in a solution of 0.10 M nicotinic acid, HC₆H₄NO₂, at 25° C? What is the pH of the solution? What is the degree of ionization of nicotinic acid? The acid-ionization constant, K_a, is 1.4 x 10^-5.

Calculating Concentrations of Species in a Weak Acid Solution Using K_a (Quadratic Formula)

What is the pH at 25° C of the solution obtained by dissolving a 5.00 grain tablet of aspirin (acetylsalicylic acid) in 0.500 L of water? The table contains 5.00 grains, or 0.325 g, of acetylsalicylic acid, HC₉H₇O₄. The acid is monoprotic, and K_a equals 3.3 x 10^-4 at 25° C.