Question

Select the stronger acid in each set. (a) Phenol \(\left(\mathrm{p} K_{\mathrm{a}} 9.95\right)\) and benzoic acid \(\left(\mathrm{p} K_{\mathrm{a}} 4.19\right)\) (b) Lactic acid \(\left(K_{\mathrm{a}} 8.4 \times 10^{-4}\right)\) and ascorbic acid \(\left(K_{\mathrm{a}} 7.9 \times 10^{-5}\right)\)

Short answer

Answer: (a) Benzoic acid is stronger than phenol. (b) Lactic acid is stronger than ascorbic acid.

Step-by-step solution

Compare pKa values of Phenol and Benzoic Acid

Compare the pKa values of phenol (pKa = 9.95) and benzoic acid (pKa = 4.19). Since benzoic acid has a lower pKa value, it is the stronger acid among the two.

Compare Ka values of Lactic Acid and Ascorbic Acid

Compare the Ka values of lactic acid (Ka = 8.4 x 10^(-4)) and ascorbic acid (Ka = 7.9 x 10^(-5)). Since lactic acid has a higher Ka value, it is the stronger acid among the two.

Report the Results

(a) In the first set, benzoic acid (pKa = 4.19) is stronger than phenol (pKa = 9.95). (b) In the second set, lactic acid (Ka = 8.4 x 10^(-4)) is stronger than ascorbic acid (Ka = 7.9 x 10^(-5)).