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Chapter 4: Problem 27

Benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\left(\mathrm{p} K_{\mathrm{a}} 4.19\right)\), is only slightly soluble in water, but its sodium salt, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-} \mathrm{Na}^{+}\), is quite soluble in water. In which solution(s) will benzoic acid dissolve? (a) Aqueous \(\mathrm{NaOH}\) (b) Aqueous \(\mathrm{NaHCO}{ }_{3}\) (c) Aqueous \(\mathrm{Na}_{2} \mathrm{CO}_{3}\)

Short Answer

Expert verified
Consider the given pKa value of benzoic acid as 4.19 and its limited solubility in water. Answer: Benzoic acid will dissolve in (a) aqueous NaOH and (c) aqueous Na2CO3 solution.

Step by step solution

01

Identify the reaction that will occur for each given solution

For each given solution, examine the possible reaction(s) between benzoic acid (C6H5COOH) and the component(s) of the solution (in this case, NaOH, NaHCO3, and Na2CO3).
02

Calculate pKb for the reaction components

Use the given pKa value for benzoic acid (4.19) and the relationship between pKa, pKb, and Kw (where Kw = 14) to calculate the pKb for each of the reaction components in the given solutions.
03

Compare the pKa of benzoic acid with the calculated pKb values

For each given solution, compare the pKa of benzoic acid with the respective pKb values. If the pKb value for the component in the solution is larger than the pKa of benzoic acid, the benzoate ion (C6H5COO-) will be formed, and benzoic acid will dissolve in that solution.
04

Determine the solubility of benzoic acid in each solution

Based on the comparison of pKa and pKb values for each given solution, determine whether benzoic acid is soluble in (a) aqueous NaOH, (b) aqueous NaHCO3, and (c) aqueous Na2CO3.
05

Summarize the findings

Summarize the results of the pKa and pKb value comparisons for each of the given solutions and present the overall finding of the solubility of benzoic acid in each solution.

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Most popular questions from this chapter

Glutamic acid is another of the amino acids found in proteins (Chapter 27). Glutamic acid has two carboxyl groups, one with \(\mathrm{p} K_{\mathrm{a}} 2.10\) and the other with \(\mathrm{p} K_{\mathrm{a}} 4.07\). [NH3+]C(CCC(=O)O)C(=O)O Glutamic acid (a) Which carboxyl group has which \(\mathrm{p} K_{\mathrm{a}}\) ? (b) Account for the fact that one carboxyl group is a considerably stronger acid than the other carboxyl group.

Offer an explanation for the following observations. (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\)is a stronger acid than \(\mathrm{NH}_{4}^{+}\). (b) Nitric acid, \(\mathrm{HNO}_{3}\), is a stronger acid than nitrous acid, \(\mathrm{HNO}_{2}\). (c) Ethanol and water have approximately the same acidity. (d) Trifluoroacetic acid, \(\mathrm{CF}_{3} \mathrm{COOH}\), is a stronger acid than trichloroacetic acid, \(\mathrm{CCl}_{3} \mathrm{COOH}\).

For each pair of molecules or ions, select the stronger base and write its Lewis structure. (a) \(\mathrm{CH}_{3} \mathrm{~S}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (b) \(\mathrm{CH}_{3} \mathrm{NH}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)or \(\mathrm{OH}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)or \(\mathrm{H}^{-}\) (e) \(\mathrm{NH}_{3}\) or \(\mathrm{OH}^{-}\) (f) \(\mathrm{NH}_{3}\) or \(\mathrm{H}_{2} \mathrm{O}\) (g) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)or \(\mathrm{HCO}_{3}^{-}\) (h) \(\mathrm{HSO}_{4}^{-}\)or \(\mathrm{OH}^{-}\) (i) \(\mathrm{OH}^{-}\)or \(\mathrm{Br}^{-}\)

For each value of \(K_{\mathrm{a}}\), calculate the corresponding value of \(\mathrm{p} K_{\mathrm{a}}\). Which compound is the stronger acid? (a) Acetic acid, \(K_{\mathrm{a}}=1.74 \times 10^{-5}\) (b) Chloroacetic acid, \(K_{\mathrm{a}}=1.38 \times 10^{-3}\)

Complete a net ionic equation for each proton-transfer reaction using curved arrows to show the flow of electron pairs in each reaction. Label the original acid and its conjugate base; then label the original base and its conjugate acid. (a) \(\mathrm{NH}_{4}{ }^{+}+\mathrm{OH}^{-} \rightleftharpoons\) (b) \(\mathrm{CH}_{3} \mathrm{COO}^{-}+\mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+} \rightleftharpoons\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}+\mathrm{NH}_{4}^{+} \rightleftharpoons\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+}+\mathrm{OH}^{-} \rightleftharpoons\)
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