Chapter 16

Classify each of the following species as a Lewis acid or a Lewis base: (a) \(\mathrm{CO}_{2},(\mathrm{~b}) \mathrm{H}_{2} \mathrm{O},(\mathrm{c}) \mathrm{I}^{-},(\mathrm{d}) \mathrm{SO}_{2},(\mathrm{e}) \mathrm{NH}_{3},\) (f) \(\mathrm{OH}^{-},(\mathrm{g}) \mathrm{H}^{+},(\mathrm{h}) \mathrm{BCl}_{3}\)

Describe the following reaction in terms of the Lewis theory of acids and bases: $$ \mathrm{AlCl}_{3}(s)+\mathrm{Cl}^{-}(a q) \longrightarrow \mathrm{AlCl}_{4}^{-}(a q) $$

Which would be considered a stronger Lewis acid: (a) \(\mathrm{BF}_{3}\) or \(\mathrm{BCl}_{3},\) (b) \(\mathrm{Fe}^{2+}\) or \(\mathrm{Fe}^{3+}\) ? Explain.

All Brönsted acids are Lewis acids, but the reverse is not true. Give two examples of Lewis acids that are not Brönsted acids.

Identify the Lewis acid and the Lewis base in the following reactions: (a) \(5 \mathrm{CO}(g)+\mathrm{Fe}(s) \longrightarrow \mathrm{Fe}(\mathrm{CO})_{5}(l)\) (b) \(\mathrm{NH}_{3}(g)+\mathrm{BCl}_{3}(g) \longrightarrow \mathrm{Cl}_{3} \mathrm{BNH}_{3}(s)\) (c) \(\mathrm{Hg}^{2+}(a q)+4 \mathrm{I}^{-}(a q) \longrightarrow \mathrm{HgI}_{4}^{2-}(a q)\)

Identify the Lewis acid and the Lewis base in the following reactions: (a) \(\mathrm{AlBr}_{3}(s)+\mathrm{Br}^{-}(a q) \longrightarrow \mathrm{AlBr}_{4}^{-}(a q)\) (b) \(6 \mathrm{CO}(g)+\mathrm{Cr}(s) \longrightarrow \operatorname{Cr}(\mathrm{CO})_{6}(s)\) (c) \(\mathrm{Cu}^{2+}(a q)+4 \mathrm{CN}^{-}(a q) \longrightarrow \mathrm{Cu}(\mathrm{CN})_{4}^{2-}(a q)\)

Predict the direction that predominates in this reaction: $$ \mathrm{F}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HF}(a q)+\mathrm{OH}^{-}(a q) $$

Predict the products and tell whether the following reaction will occur to any measurable extent: $$ \mathrm{CH}_{3} \mathrm{COOH}(a q)+\mathrm{Cl}^{-}(a q) \longrightarrow $$

Calculate the \(\mathrm{pH}\) of a \(0.20 \mathrm{M}\) ammonium acetate \(\left(\mathrm{CH}_{3} \mathrm{COONH}_{4}\right)\) solution.

To which of the following would the addition of an equal volume of \(0.60 \mathrm{M} \mathrm{NaOH}\) lead to a solution having (b) \(0.30 \mathrm{M} \mathrm{HCl}\) (c) \(0.70 \mathrm{M} \mathrm{KOH}\) a lower \(\mathrm{pH}\) : (a) water, (d) \(0.40 \mathrm{M} \mathrm{NaNO}_{3}\) ?

The \(\mathrm{pH}\) of a \(0.0642-M\) solution of a monoprotic acid is 3.86. Is this a strong acid?

Like water, liquid ammonia undergoes autoionization: $$ \mathrm{NH}_{3}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{NH}_{2}^{-} $$ (a) Identify the Brönsted acids and Bronsted bases in this reaction. (b) What species correspond to \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{OH}^{-},\) and what is the condition for a neutral solution?

\(\mathrm{HA}\) and \(\mathrm{HB}\) are both weak acids although \(\mathrm{HB}\) is the stronger of the two. Will it take a larger volume of a \(0.10 \mathrm{M}\) \(\mathrm{NaOH}\) solution to neutralize \(50.0 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{HB}\) than would be needed to neutralize \(50.0 \mathrm{~mL}\) of \(0.10 \mathrm{M}\) HA?

The three common chromium oxides are \(\mathrm{CrO}, \mathrm{Cr}_{2} \mathrm{O}_{3},\) and \(\mathrm{CrO}_{3}\). If \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) is amphoteric, what can you say about the acid-base properties of \(\mathrm{CrO}\) and \(\mathrm{CrO}_{3} ?\)

Most of the hydrides of Group \(1 \mathrm{~A}\) and Group \(2 \mathrm{~A}\) metals are ionic (the exceptions are \(\mathrm{BeH}_{2}\) and \(\mathrm{MgH}_{2}\), which are covalent compounds). (a) Describe the reaction between the hydride ion \(\left(\mathrm{H}^{-}\right)\) and water in terms of a Brønsted acid-base reaction. (b) The same reaction can also be classified as a redox reaction. Identify the oxidizing and reducing agents.

A \(10.0-\mathrm{g}\) sample of white phosphorus was burned in an excess of oxygen. The product was dissolved in enough water to make \(500.0 \mathrm{~mL}\) of solution. Calculate the \(\mathrm{pH}\) of the solution at \(25^{\circ} \mathrm{C}\).

Which of the following is the stronger base: \(\mathrm{NF}_{3}\) or \(\mathrm{NH}_{3}\) ? (Hint: \(\mathrm{F}\) is more electronegative than \(\mathrm{H}\).)

Which of the following is a stronger base: \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) ? (Hint: The \(\mathrm{N}-\mathrm{H}\) bond is stronger than the \(\mathrm{P}-\mathrm{H}\) bond. \()\)

The ion product of \(\mathrm{D}_{2} \mathrm{O}\) is \(1.35 \times 10^{-15}\) at \(25^{\circ} \mathrm{C}\). (b) For (a) Calculate \(\mathrm{pD}\) where \(\mathrm{pD}=-\log \left[\mathrm{D}_{3} \mathrm{O}^{+}\right]\). what values of \(\mathrm{pD}\) will a solution be acidic in \(\mathrm{D}_{2} \mathrm{O} ?\) (c) Derive a relation between \(\mathrm{pD}\) and \(\mathrm{pOD}\).

Give an example of (a) a weak acid that contains oxygen atoms, (b) a weak acid that does not contain oxygen atoms, (c) a neutral molecule that acts as a Lewis acid, (d) a neutral molecule that acts as a Lewis base, (e) a weak acid that contains two ionizable \(\mathrm{H}\) atoms, (f) a conjugate acid-base pair, both of which react with \(\mathrm{HCl}\) to give carbon dioxide gas.

\(\mathrm{HF}\) is a weak acid, but its strength increases with concentration. Explain. (Hint: \(\mathrm{F}^{-}\) reacts with \(\mathrm{HF}\) to form \(\mathrm{HF}_{2}^{-}\). The equilibrium constant for this reaction is 5.2 at \(25^{\circ} \mathrm{C} .\) )

When chlorine reacts with water, the resulting solution is weakly acidic and reacts with \(\mathrm{AgNO}_{3}\) to give a white precipitate. Write balanced equations to represent these reactions. Explain why manufacturers of household bleaches add bases such as \(\mathrm{NaOH}\) to their products to increase their effectiveness.

When the concentration of a strong acid is not substantially higher than \(1.0 \times 10^{-7} M\), the ionization of water must be taken into account in the calculation of the solution's \(\mathrm{pH}\). (a) Derive an expression for the \(\mathrm{pH}\) of a strong acid solution, including the contribution to \(\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\) from \(\mathrm{H}_{2} \mathrm{O}\). (b) Calculate the pH of a \(1.0 \times 10^{-7} M \mathrm{HCl}\) solution.

\(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid, but \(\mathrm{HSO}_{4}^{-}\) is a weak acid. Account for the difference in strength of these two related species.

Calculate the concentrations of all the species in a \(0.100 \mathrm{M} \mathrm{Na}_{2} \mathrm{CO}_{3}\) solution.

A \(20.27-\mathrm{g}\) sample of a metal carbonate \(\left(\mathrm{MCO}_{3}\right)\) is combined with \(500 \mathrm{~mL}\) of a \(1.00 \mathrm{M} \mathrm{HCl}\) solution. The excess \(\mathrm{HCl}\) acid is then neutralized by \(32.80 \mathrm{~mL}\) of \(0.588 \mathrm{M} \mathrm{NaOH}\). Identify \(\underline{\mathrm{M}}\)

You are given two beakers, one containing an aqueous solution of strong acid (HA) and the other an aqueous solution of weak acid (HB) of the same concentration. Describe how you would compare the strengths of these two acids by (a) measuring the \(\mathrm{pH},\) (b) measuring electrical conductance, and (c) studying the rate of hydrogen gas evolution when these solutions are combined with an active metal such as \(\mathrm{Mg}\) or \(\mathrm{Zn}\).

Use Le Châtelier's principle to predict the effect of the following changes on the extent of hydrolysis of sodium nitrite \(\left(\mathrm{NaNO}_{2}\right)\) solution: (a) \(\mathrm{HCl}\) is added, (b) \(\mathrm{NaOH}\) is added, (c) \(\mathrm{NaCl}\) is added, (d) the solution is diluted.

A \(0.400 M\) formic acid (HCOOH) solution freezes at \(-0.758^{\circ} \mathrm{C}\). Calculate the \(K_{a}\) of the acid at that temperature. (Hint: Assume that molarity is equal to molality. Carry out your calculations to three significant figures and round off to two for \(K_{\mathrm{a}}\).)

Which of the following does not represent a Lewis acidbase reaction? (a) \(\mathrm{H}_{2} \mathrm{O}+\mathrm{H}^{+} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(\mathrm{NH}_{3}+\mathrm{BF}_{3} \longrightarrow \mathrm{H}_{3} \mathrm{NBF}_{3}\) (c) \(\mathrm{PF}_{3}+\mathrm{F}_{2} \longrightarrow \mathrm{PF}_{5}\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow \mathrm{Al}(\mathrm{OH})_{4}^{-}\)

Describe the hydration of \(\mathrm{SO}_{2}\) as a Lewis acid-base reaction.

Both the amide ion \(\left(\mathrm{NH}_{2}^{-}\right)\) and the nitride ion \(\left(\mathrm{N}^{3-}\right)\) are stronger bases than the hydroxide ion and hence do not exist in aqueous solutions. (a) Write equations showing the reactions of these ions with water, and identify the Brönsted acid and base in each case. (b) Which of the two is the stronger base?

Determine whether each of the following statements is true or false. If false, explain why the statement is wrong. (a) All Lewis acids are Brønsted acids. (b) The conjugate base of an acid always carries a negative charge. (c) The percent ionization of a base increases with its concentration in solution. (d) A solution of barium fluoride is acidic.

How many milliliters of a strong monoprotic acid solution at \(\mathrm{pH}=4.12\) must be added to \(528 \mathrm{~mL}\) of the same acid solution at \(\mathrm{pH}=5.76\) to change its \(\mathrm{pH}\) to 5.34? Assume that the volumes are additive.

The disagreeable odor of fish is mainly due to organic compounds \(\left(\mathrm{RNH}_{2}\right)\) containing an amino group, \(-\mathrm{NH}_{2}\), where \(\mathrm{R}\) is the rest of the molecule. Amines are bases just like ammonia. Explain why putting some lemon juice on fish can greatly reduce the odor.

Explain the action of smelling salt, which is ammonium carbonate \(\left[\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\right]\). (Hint: The thin film of aqueous solution that lines the nasal passages is slightly basic.)

A typical reaction between an antacid and the hydrochloric acid in gastric juice is \(\mathrm{NaHCO}_{3}(s)+\mathrm{HCl}(a q) \rightleftharpoons \mathrm{NaCl}(a q)+\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CO}_{2}(g)\). Calculate the volume (in liters) of \(\mathrm{CO}_{2}\) generated from \(0.350 \mathrm{~g}\) of \(\mathrm{NaHCO}_{3}\) and excess gastric juice at \(1.00 \mathrm{~atm}\) and \(37.0^{\circ} \mathrm{C}\).

Novocaine, used as a local anesthetic by dentists, is a weak base \(\left(K_{\mathrm{b}}=8.91 \times 10^{-6}\right) .\) What is the ratio of the concentration of the base to that of its acid in the blood plasma \((\mathrm{pH}=7.40)\) of a patient? (As an approximation, use the \(K_{\mathrm{a}}\) values at \(25^{\circ} \mathrm{C}\).)

Hemoglobin (Hb) is a blood protein that is responsible for transporting oxygen. It can exist in the protonated form as \(\mathrm{HbH}^{+}\). The binding of oxygen can be represented by the simplified equation: $$ \mathrm{HbH}^{+}+\mathrm{O}_{2} \rightleftharpoons \mathrm{HbO}_{2}+\mathrm{H}^{+} $$ (a) What form of hemoglobin is favored in the lungs where oxygen concentration is highest? (b) In body tissues, where the cells release carbon dioxide produced by metabolism, the blood is more acidic due to the formation of carbonic acid. What form of hemoglobin is favored under this condition? (c) When a person hyperventilates, the concentration of \(\mathrm{CO}_{2}\) in his or her blood decreases. How does this action affect the given equilibrium? Frequently a person who is hyperventilating is advised to breathe into a paper bag. Why does this action help the individual?

Tooth enamel is largely hydroxyapatite \(\left[\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH}\right]\). When it dissolves in water (a process called demineralization), it dissociates as follows: $$ \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH} \longrightarrow 5 \mathrm{Ca}^{2+}+3 \mathrm{PO}_{4}^{3-}+\mathrm{OH}^{-} $$ The reverse process, called remineralization, is the body's natural defense against tooth decay. Acids produced from food remove the \(\mathrm{OH}^{-}\) ions and thereby weaken the enamel layer. Most toothpastes contain a fluoride compound such as \(\mathrm{NaF}\) or \(\mathrm{SnF}_{2}\). What is the function of these compounds in preventing tooth decay?

Tooth enamel is largely hydroxyapatite \(\left[\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH}\right]\). When it dissolves in water (a process called demineralization), it dissociates as follows: $$ \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH} \longrightarrow 5 \mathrm{Ca}^{2+}+3 \mathrm{PO}_{4}^{3-}+\mathrm{OH}^{-} $$ The reverse process, called remineralization, is the body's natural defense against tooth decay. Acids produced from food remove the \(\mathrm{OH}^{-}\) ions and thereby weaken the enamel layer. Most toothpastes contain a fluoride compound such as \(\mathrm{NaF}\) or \(\mathrm{SnF}_{2}\). What is the function of these compounds in preventing tooth decay?

The atmospheric sulfur dioxide \(\left(\mathrm{SO}_{2}\right)\) concentration over a certain region is 0.12 ppm by volume. Calculate the \(\mathrm{pH}\) of the rainwater due to this pollutant. Assume that the dissolution of \(\mathrm{SO}_{2}\) does not affect its pressure. $$ \left(K_{\mathrm{a}} \text { for } \mathrm{H}_{2} \mathrm{SO}_{3}=1.3 \times 10^{-2} .\right) $$

About half of the hydrochloric acid produced annually in the United States ( 3.0 billion pounds) is used in metal pickling. This process involves the removal of metal oxide layers from metal surfaces to prepare them for coating. (a) Write the overall and net ionic equations for the reaction between iron(III) oxide, which represents the rust layer over iron, and HCl. Identify the Brønsted acid and base. (b) Hydrochloric acid is also used to remove scale (which is mostly \(\mathrm{CaCO}_{3}\) ) from water pipes. Hydrochloric acid reacts with calcium carbonate in two stages; the first stage forms the bicarbonate ion, which then reacts further to form carbon dioxide. Write equations for these two stages and for the overall reaction. (c) Hydrochloric acid is used to recover oil from the ground. It dissolves rocks (often \(\mathrm{CaCO}_{3}\) ) so that the oil can flow more easily. In one process, a 15 percent (by mass) HCl solution is injected into an oil well to dissolve the rocks. If the density of the acid solution is \(1.073 \mathrm{~g} / \mathrm{mL},\) what is the \(\mathrm{pH}\) of the solution?

At \(28^{\circ}\) and 0.982 atm, gaseous compound HA has a density of \(1.16 \mathrm{~g} / \mathrm{L}\). A quantity of \(2.03 \mathrm{~g}\) of this compound is dissolved in water and diluted to exactly \(1 \mathrm{~L}\). If the \(\mathrm{pH}\) of the solution at \(25^{\circ} \mathrm{C}\) is 5.22 (due to ionization of \(\mathrm{HA}\) ), calculate the \(K_{\mathrm{a}}\) of the acid.

(a) Use VSEPR to predict the geometry of the hydronium ion \(\left(\mathrm{H}_{3} \mathrm{O}^{+}\right) .\) (b) The \(\mathrm{O}\) atom in \(\mathrm{H}_{2} \mathrm{O}\) has two lone pairs and in principle can accept two \(\mathrm{H}^{+}\) ions. Explain why the species \(\mathrm{H}_{4} \mathrm{O}^{2+}\) does not exist. What would be its geometry if it did exist?