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Question 18.1: Classifying Acid and Base Strength from the Chemical Formula......

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

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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)
              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.

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