AP Chemistry 8.7: Protonation and pKa

Practice AP Chemistry questions on predicting whether a weak acid or base is protonated or deprotonated by comparing solution pH with pKa.

[Maximum number: 1]

A student reacts 0.300 g of methyl salicylate $\left(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}_{3}\right)$ with a stoichiometric amount of a strong base. This product is then acidified to produce salicylic acid crystals $\left(\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{3}\right)$ .

When the pH of the titration mixture is 4.00, is there a higher concentration of the weak acid, $\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{3}$ , or its conjugate base, $\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}{ }^{-}$ , in the flask? Justify your answer.

For the correct answer and a valid justification, consistent with part (e):
Accept one of the following:
- The conjugate base, $\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}{ }^{-}$ . When pH=4, the titration is beyond the half-
equivalence point, where $\left[\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{3}\right]=\left[\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}{ }^{-}\right]$ . Thus, $\left[\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}{ }^{-}\right]$ must be
greater than $\left[\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{3}\right]$ .
- The conjugate base, $\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}{ }^{-}$ . Because the pH of the solution is greater than the
$\mathrm{p} K_{a}$ of the acid, the majority of the molecules will be deprotonated.

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8.7.A—Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the… question 1