Q. 18.1

Classifying Acid and Base Strength from the Chemical Formula

Problem Classify each of the following compounds as a strong acid, weak acid, strong base, or weak base, and write the $K_a$ expression for any weak acid:

(a) KOH          (b) $(CH_3)_2CHCOOH$           (c) $H_2SeO_4$          (d) $(CH_3)_2CHNH_2$

Verified Solution

Plan We examine the formula and classify each compound as acid or base, using the text descriptions. Particular points to note for acids are the numbers of O atoms relative to ionizable H atoms and the presence of the —COOH group. For bases, we note the nature of the cation or the presence of an N atom that has a lone pair.

Solution (a) Strong base: KOH is one of the Group 1A(1) hydroxides.
(b) Weak acid: $(CH_3)_2CHCOOH$ is a carboxylic acid, as indicated by the —COOH group; the dissociation reaction is:

$(CH_3)_2CHCOOH(aq) + H_2O(l) \xrightleftharpoons[] H_3O^+(aq) + (CH_3)_2CHCOO^−(aq)$
and
$Ka = \frac{[H_3O^+][(CH_3)_2CHCOO^−]}{[(CH_3)_2CHCOOH]}$
(c) Strong acid: $H_2SeO_4$ is an oxoacid in which the number of O atoms exceeds the number of ionizable protons by 2.
(d) Weak base: $(CH_3)_2CHNH_2$ has a lone electron pair on the N and is an amine.