Equilibria of Acids and Bases

The acid-dissociation constant, Ka, for benzoic acid is 6.5 × 10-5.

1.5x10^-10

Determine the pH of a 0.0500 M solution of potassium hydroxide (KOH).

12.7

The base-dissociation constant, Kb, for aniline is 4.3 × 10-10.

2.3x10^-5

Determine the pH of a 0.05 M solution of hydrocyanic acid (HCN). Hydrocyanic acid has an acid-dissociation constant of 4.9 × 10-10.

5.3

At the beginning of this lesson, there was a question about two acids. Vinegar, which contains acetic acid, is used in foods and has few safety concerns. Hydrochloric acid is used in chemistry labs and requires the use of safety goggles and gloves. Why do the safety concerns for these two acids differ?

Acetic acid is a weak acid, and hydrochloric acid is a strong acid. As a weak acid, acetic acid does not dissociate much. As a strong acid, hydrochloric acid dissociates completely. The stronger the acid, the more hydrogen ions will be in a solution at a given concentration. Because they donate hydrogen ions, acids can react with other materials that accept hydrogen ions. A stronger acid will be more reactive than a weak acid at the same concentration.

Which will you use to calculate the base-dissociation constant, Kb, for the conjugate base of benzoic acid?

Ka x Kb = Kw

which has the weakest conjugate acid

dimethylamine

which is the strongest acid

formic

which has the weakest base

hydrazine

which has the strongest conjugate base

phenol