Chapter 17

Carbon monoxide forms complexes with low-valent metals. For example, \(\mathrm{Ni}(\mathrm{CO})_{4}\) and \(\mathrm{Fe}(\mathrm{CO})_{5}\) are well known. CO also forms complexes with the iron(II) ion in hemoglobin, which prevents the hemoglobin from acting in its normal way. Is CO a Lewis acid or a Lewis base?

Trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N},\) is a common reagent. It interacts readily with diborane gas, \(\mathrm{B}_{2} \mathrm{H}_{6}\). The latter dissociates to \(\mathrm{BH}_{3},\) and this forms a complex with the amine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N} \rightarrow \mathrm{BH}_{3} .\) Is the \(\mathrm{BH}_{3}\) fragment a Lewis acid or a Lewis base?

Which should be the stronger acid, HOCN or HCN? Explain briefly. (In HOCN, the \(\mathrm{H}^{+}\) ion is attached to the O atom of the OCN " ion.)

Which should be the stronger Bronsted acid, \(\left[\mathrm{V}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) or \(\left[\mathrm{V}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+} ?\)

About this time, you may be wishing you had an aspirin. Aspirin is an organic acid (page 507 ) with a \(K_{\mathrm{a}}\) of \(3.27 \times\) \(10^{-4}\) for the reaction \(\mathrm{HC}_{9} \mathrm{H}_{7} \mathrm{O}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftarrows \mathrm{C}_{9} \mathrm{H}_{7} \mathrm{O}_{4}^{-}(\mathrm{aq})+\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})\) If you have two tablets, each containing \(0.325 \mathrm{g}\) of aspirin (mixed with a neutral "binder" to hold the tablet together), and you dissolve them in a glass of water to give 225 mL of solution, what is the pH of the solution?

\(\begin{aligned} &\text { Consider the following ions: } \mathrm{NH}_{4}^{+}, \mathrm{CO}_{3}^{2-}, \mathrm{Br}^{-}, \mathrm{S}^{2-}, \text { and }\\\ &\mathrm{ClO}_{4}^{-} \end{aligned}\) (a) Which of these ions might lead to an acidic solution and which might lead to a basic solution? (b) Which of these anions will have no effect on the pH of an aqueous solution? (c) Which ion is the strongest base? (d) Write a chemical equation for the reaction of each basic anion with water.

You have \(0.010 \mathrm{M}\) solutions of benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\) \(\left(K_{\mathrm{a}}=6.3 \times 10^{-5}\right)\) and 4 -chlorobenzoic acid, \(\mathrm{ClC}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{H}\) \(\left(K_{\mathrm{a}}=1.0 \times 10^{-4}\right) .\) Which solution will have the higher pH?

Place the following acids in order of (i) increasing strength and (ii) increasing \(\mathrm{pH}\). Assume you have a \(0.10 \mathrm{M}\) solution of each acid. (a) 4 -chlorobenzoic acid, \(\mathrm{ClC}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{H}, K_{\mathrm{a}}=1.0 \times 10^{-4}\) (b) bromoacetic acid, \(\mathrm{BrCH}_{2} \mathrm{CO}_{2} \mathrm{H}, K_{\mathrm{a}}=1.3 \times 10^{-3}\) (c) trimethylammonium ion, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NH}^{+}, K_{\mathrm{a}}=1.6 \times 10^{-10}\)

A monoprotic acid HX has \(K_{a}=1.3 \times 10^{-3} .\) Calculate the equilibrium concentration of \(\mathrm{HX}\) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) and the \(\mathrm{pH}\) for a \(0.010 \mathrm{M}\) solution of the acid.

Calcium hydroxide, \(\operatorname{Ca}(\mathrm{OH})_{2},\) is almost insoluble in water \(=\) only \(0.50 \mathrm{g}\) can be dissolved in \(1.0 \mathrm{L}\) of water at \(25^{\circ} \mathrm{C} .\) If the dissolved substance is completely dissociated into its constituent ions, what is the pH of a saturated solution?

The anilinium ion, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+},\) is the conjugate acid of the weak organic base aniline. If the anilinium ion has a \(\mathrm{p} K_{\mathrm{a}}\) of \(4.60,\) what is the \(\mathrm{pH}\) of a \(0.080 \mathrm{M}\) solution of anilinium hydrochloride, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{Cl}\) ?

The base ethylamine \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)\) has a \(K_{\mathrm{b}}\) of \(4.3 \times 10^{-4}\) A closely related base, ethanolamine (HOCH,CH \(_{2} \mathrm{NH}_{2}\) ), has a \(K_{\mathrm{b}}\) of \(3.2 \times 10^{-5}\) (a) Which of the two bases is stronger? (b) Calculate the \(\mathrm{pH}\) of a \(0.10 \mathrm{M}\) solution of the stronger base.

Chloroacetic acid, \(\mathrm{ClCH}_{2} \mathrm{CO}_{2} \mathrm{H}\), is a moderately weak acid \(\left(K_{\mathrm{a}}=1.40 \times 10^{-3}\right) .\) If you dissolve \(94.5 \mathrm{mg}\) of the acid in water to give 125 mL of solution, what is the pH of the solution?

For each of the following salts, predict whether a \(0.1 \mathrm{M}\) solution has a pH less than, equal to, or greater than 7 (a) \(\mathrm{NaHSO}_{4}\) (b) \(\mathrm{NH}_{4} \mathrm{Br}\) (c) \(\mathrm{KClO}_{4}\) (d) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (e) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}\) (f) \(\mathrm{NaNO}_{3}\) (g) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (h) LiBr (i) \(\mathrm{FeCl}_{3}\)

Given the following solutions: (a) \(0.1 \mathrm{M} \mathrm{NH}_{3}\) (e) \(0.1 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}\) (b) \(0.1 \mathrm{M} \mathrm{Na}_{2} \mathrm{CO}_{3}\) (f) \(0.1 \mathrm{M} \mathrm{NaCH}_{3} \mathrm{CO}_{2}\) (c) \(0.1 \mathrm{M} \mathrm{NaCl}\) (g) \(0.1 \mathrm{M} \mathrm{NH}_{4} \mathrm{CH}_{3} \mathrm{CO}_{2}\) (d) \(0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (i) Which of the solutions are acidic? (ii) Which of the solutions are basic? (iii) Which of the solutions is most acidic?

To what volume should \(1.00 \times 10^{2} \mathrm{mL}\) of any weak acid, HA, with a concentration \(0.20 \mathrm{M}\) be diluted to double the percentage ionization?

Arrange the following \(0.10 \mathrm{M}\) solutions in order of increasing pH. (a) \(\mathrm{NaCl}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\mathrm{HCl}\) $$\begin{aligned} &\text { (d) } \mathrm{NaCH}_{3} \mathrm{CO}_{2}\\\ &\text { (e) } \mathrm{KOH} \end{aligned}$$

Calculate the pH of the solution that results from mixing \(25.0 \mathrm{mL}\) of \(0.14 \mathrm{M}\) formic acid and \(50.0 \mathrm{mL}\) of \(0.070 \mathrm{M}\) sodium hydroxide.

Iodine, \(I_{2}\), is much more soluble in a water solution of potassium iodide, KI, than it is in pure water. The anion found in solution is \(I_{3}^{-}\) (a) Draw an electron dot structure for \(\mathrm{I}_{3}^{-}\). (b) Write an equation for this reaction, indicating the Lewis acid and the Lewis base.

Why can water be both a Bronsted base and a Lewis base? Can water be a Bronsted acid? A Lewis acid?

The nickel(II) ion exists as \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) in aqueous solution. Why is such a solution acidic? As part of your answer include a balanced equation depicting what happens when \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) interacts with water.

Describe an experiment that will allow you to place the following three bases in order of increasing base strength: \(\mathrm{NaCN}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{Na}_{2} \mathrm{CO}_{3}\)

Which should be the stronger acid: \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) or \(\mathrm{H}_{2} \mathrm{SeO}_{3} ?\) Why? Describe an experiment by which you could confirm your prediction.

You prepare a \(0.10 \mathrm{M}\) solution of \(\mathrm{HCN}\). What molecules and ions exist in this solution? List them in order of decreasing concentration.

A You mix \(30.0 \mathrm{mL}\) of \(0.15 \mathrm{M} \mathrm{NaOH}\) with \(30.0 \mathrm{mL}\) of \(0.15 \mathrm{M}\) acetic acid. What molecules and ions exist in this solution? List them in order of decreasing concentration.

Perchloric acid behaves as an acid, even when it is dissolved in sulfuric acid. (a) Write a balanced equation showing how perchloric acid can transfer a proton to sulfuric acid. (b) Draw a Lewis electron dot structure for sulfuric acid. How can sulfuric acid function as a base?

You purchase a bottle of water. On checking its pH, you find that it is not neutral as you might have expected. Instead, it is slightly acidic. Why?

A You have three solutions labeled \(A, B,\) and \(C .\) You know only that each contains a different cation-Na \(^{+}\) \(\mathrm{NH}_{4}^{+},\) or \(\mathrm{H}^{+}\). Each has an anion that does not contribute to the solution \(\mathrm{pH}\) (e.g., \(\mathrm{Cl}^{-}\) ). You also have two other solutions, Y and Z, each containing a different anion, \(\mathrm{Cl}^{-}\) or \(\mathrm{OH}^{-},\) with a cation that does not influence solution \(\mathrm{pH}\left(\text { e.g. }, \mathrm{K}^{+}\right) .\) If equal amounts of \(\mathrm{B}\) and \(\mathrm{Y}\) are mixed, the result is an acidic solution. Mixing A and Z gives a neutral solution, whereas \(\mathrm{B}\) and \(\mathrm{Z}\) give a basic solution. Identify the five unknown solutions. (Adapted from D. H. Barouch: Voyages in Conceptual Chemistry, Boston, Jones and Bartlett, \(1997 .)\) (TABLE CAN'T COPY)

A hydrogen atom in the organic base pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\), can be substituted by various atoms or groups to give \(\mathrm{XC}_{5} \mathrm{H}_{4} \mathrm{N},\) where \(\mathrm{X}\) is an atom such as \(\mathrm{Cl}\) or a group such as \(\mathrm{CH}_{3}\). The following table gives \(K_{\mathrm{a}}\) values for the conjugate acids of a variety of substituted pyridines. $$ \begin{array}{ll} \hline \text { Atom or Group } \mathrm{X} & K_{\mathrm{a}} \text { of Conjugate Acid } \\ \hline \mathrm{N} 0_{2} & 5.9 \times 10^{-2} \\ \mathrm{Cl} & 1.5 \times 10^{-4} \\ \mathrm{H} & 6.8 \times 10^{-6} \\ \mathrm{CH}_{3} & 1.0 \times 10^{-6} \\ \hline \end{array} $$ (a) Suppose each conjugate acid is dissolved in sufficient water to give a 0.050 M solution. Which solution would have the highest pH? The lowest pH? (b) Which of the substituted pyridines is the strongest Bronsted base? Which is the weakest Brønsted base?