Chapter 16

Write an equation for the ionization of hydrogen cyanide, \(\mathrm{HCN},\) in aqueous solution. What is the equilibrium expression \(K_{a}\) for this acid ionization?

Briefly describe two methods for determining \(K_{a}\) for a weak acid.

Describe how the degree of ionization of a weak acid changes as the concentration increases.

Consider a solution of \(0.0010 \mathrm{MHF}\left(K_{a}=6.8 \times 10^{-4}\right)\). In solving for the concentrations of species in this solution, could you use the simplifying assumption in which you neglect \(x\) in the denominator of the equilibrium equation? Explain. x

Phosphorous acid, \(\mathrm{H}_{2} \mathrm{PHO}_{3},\) is a diprotic acid. Write equations for the acid ionizations. Write the expressions for \(K_{a 1}\) and \(K_{a 2}\)

Write the equation for the ionization of aniline, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2},\) in aqueous solution. Write the expression for \(K_{b}\)

What is meant by the common-ion effect? Give an example.

The \(\mathrm{pH}\) of \(0.10 \mathrm{M} \mathrm{CH}_{3} \mathrm{NH}_{2}\) (methylamine) is 11.8 . When the chloride salt of methylamine, \(\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Cl}\), is added to this solution, does the pH increase or decrease? Explain, using Le Châtelier's principle and the commonion effect.

Describe the pH changes that occur during the titration of a weak base by a strong acid. What is meant by the term equivalence point?

Which of the following salts would produce the most basic aqueous solution? a. \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) b. \(\mathrm{LiI}\) c. \(\mathrm{KCl}\) d. \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) e. KClO

If you mix \(0.10 \mathrm{~mol}\) of \(\mathrm{NH}_{3}\) and \(0.10 \mathrm{~mol}\) of \(\mathrm{HCl}\) in a bucket of water, you would expect the resulting solution to be a. very basic b. slightly basic c. neutral d. slightly acidic e. very acidic

Hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{~S},\) is a very weak diprotic acid. In a \(0.10 M\) solution of this acid, which of the following would you expect to find in the highest concentration? a. \(\mathrm{H}_{2} \mathrm{~S}\) b. \(\mathrm{H}_{3} \mathrm{O}^{+}\) c. HS \(^{-}\) d. \(S^{2}-\) e. OH

If \(20.0 \mathrm{~mL}\) of a \(0.10 \mathrm{M} \mathrm{NaOH}\) solution is added to a 30.0 -mL sample of a \(0.10 M\) weak acid, HA, what is the \(\mathrm{pH}\) of the resulting solution? \(\left(K_{a}=1.8 \times 10^{-5}\right.\) for \(\left.\mathrm{HA}\right)\) a. 2.87 b. 2.74 c. 4.74 d. 5.05 e. 8.73

a. For each of the following salts, write the reaction that occurs when it dissociates in water: \(\mathrm{NaCl}(s), \mathrm{NaCN}(s)\) \(\mathrm{KClO}_{2}(s), \mathrm{NH}_{4} \mathrm{NO}_{3}(s), \mathrm{K} \operatorname{Br}(a q),\) and \(\mathrm{NaF}(s)\) b. Consider each of the reactions that you wrote above, and identify the aqueous ions that could be proton donors (acids) or proton acceptors (bases). Briefly explain how you decided which ions to choose. c. For each of the acids and bases that you identified in part \(\mathrm{b}\), write the chemical reaction it can undergo in aqueous solution (its reaction with water). d. Are there any reactions that you have written above that you anticipate will occur to such an extent that the \(\mathrm{pH}\) of the solution will be affected? As part of your answer, be sure to explain how you decided. e. Assume that in each case above, \(0.01 \mathrm{~mol}\) of the salt was dissolved in enough water at \(25^{\circ} \mathrm{C}\) to make \(1.0 \mathrm{~L}\) of solution. In each case, what additional information would you need in order to calculate the pH? If there are cases where no additional information is required, be sure to state that as well. f. Say you take \(0.01 \mathrm{~mol}\) of \(\mathrm{NH}_{4} \mathrm{CN}\) and dissolve it in enough water at \(25^{\circ} \mathrm{C}\) to make \(1.0 \mathrm{~L}\) of solution. Using chemical reactions and words, explain how you would go about determining what effect this salt will have on the \(\mathrm{pH}\) of the solution. Be sure to list any additional information you would need to arrive at an answer.

a. When \(0.10 \mathrm{~mol}\) of the ionic solid \(\mathrm{NaX},\) where \(\mathrm{X}\) is an unknown anion, is dissolved in enough water to make \(1.0 \mathrm{~L}\) of solution, the \(\mathrm{pH}\) of the solution is 9.12. When \(0.10 \mathrm{~mol}\) of the ionic solid \(\mathrm{ACl}\), where \(\mathrm{A}\) is an unknown cation, is dissolved in enough water to make \(1.0 \mathrm{~L}\) of solution, the \(\mathrm{pH}\) of the solution is 7.00. What would be the \(\mathrm{pH}\) of \(1.0 \mathrm{~L}\) of solution that contained 0.10 mol of \(A X ?\) Be sure to document how you arrived at your answer. b. In the AX solution prepared above, is there any \(\mathrm{OH}^{-}\) present? If so, compare the \(\left[\mathrm{OH}^{-}\right]\) in the solution to the \(\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\) c. From the information presented in part a, calculate \(K_{b}\) for the \(\mathrm{X}^{-}(a q)\) anion and \(K_{a}\) for the conjugate acid of \(\mathrm{X}^{-}(a q)\) d. To \(1.0 \mathrm{~L}\) of solution that contains \(0.10 \mathrm{~mol}\) of \(\mathrm{AX}\) you add 0.025 mol of \(\mathrm{HCl}\). How will the \(\mathrm{pH}\) of this solution compare to that of the solution that contained only \(\mathrm{NaX}\) ? Use chemical reactions as part of your explanation; you do not need to solve for a numerical answer. e. Another \(1.0 \mathrm{~L}\) sample of solution is prepared by mixing \(0.10 \mathrm{~mol}\) of \(\mathrm{AX}\) and \(0.10 \mathrm{~mol}\) of \(\mathrm{HCl}\). The \(\mathrm{pH}\) of the resulting solution is found to be 3.12 . Explain why the \(\mathrm{pH}\) of this solution is 3.12 . f. Finally, consider a different 1.0 - \(\mathrm{L}\) sample of solution that contains \(0.10 \mathrm{~mol}\) of \(\mathrm{AX}\) and \(0.1 \mathrm{~mol}\) of \(\mathrm{NaOH}\). The \(\mathrm{pH}\) of this solution is found to be 13.00 . Explain why the \(\mathrm{pH}\) of this solution is 13.00 . g. Some students mistakenly think that a solution that contains \(0.10 \mathrm{~mol}\) of \(\mathrm{AX}\) and \(0.10 \mathrm{~mol}\) of \(\mathrm{HCl}\) should have a pH of 1.00 . Can you come up with a reason why students have this misconception? Write an approach that you would use to help these students understand what they are doing wrong.

You have 0.10 -mol samples of three acids identified simply as HX, HY, and HZ. For each acid, you make up \(0.10 \mathrm{M}\) solutions by adding sufficient water to each of the acid samples. When you measure the \(\mathrm{pH}\) of these samples, you find that the \(\mathrm{pH}\) of \(\mathrm{HX}\) is greater than the \(\mathrm{pH}\) of \(\mathrm{HY}\), which in turn is greater than the \(\mathrm{pH}\) of \(\mathrm{HZ}\). a. Which of the acids is the least ionized in its solution? b. Which acid has the largest \(K_{a}\) ?

What reaction occurs when each of the following is dissolved in water? a. HF b. NaF c. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) d. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{Cl}\)

You have the following solutions, all of the same molar concentration: \(\mathrm{KBr}, \mathrm{HBr}, \mathrm{CH}_{3} \mathrm{NH}_{2},\) and \(\mathrm{NH}_{4} \mathrm{Cl}\). Rank them from the lowest to the highest hydroxide-ion concentration.

Rantidine is a nitrogen base that is used to control stomach acidity by suppressing the stomach's production of hydrochloric acid. The compound is present in Zantac \(^{\mathbb{B}}\) as the chloride salt (rantidinium chloride; also called rantidine hydrochloride). Do you expect a solution of rantidine hydrochloride to be acidic, basic, or neutral? Explain by means of a general chemical equation.

A chemist prepares dilute solutions of equal molar concentrations of \(\mathrm{NH}_{3}, \mathrm{NH}_{4} \mathrm{Br}, \mathrm{NaF},\) and \(\mathrm{NaCl} .\) Rank these solutions from highest \(\mathrm{pH}\) to lowest \(\mathrm{pH}\).

\( \mathrm{~A}\) friend of yours has performed three titrations: strong acid with a strong base, weak acid with a strong base, and weak base with a strong acid. He hands you the three titration curves, saying he has forgotten which is which. What attributes of the curves would you look at to identify each curve correctly?

Write chemical equations for the acid ionizations of each of the following weak acids (express these in terms of \(\left.\mathrm{H}_{3} \mathrm{O}^{+}\right)\) a. \(\mathrm{HBrO}\) (hypobromous acid) b. \(\mathrm{HClO}_{2}\) (chlorous acid) c. \(\mathrm{HNO}_{2}\) (nitrous acid) d. HCN (hydrocyanic acid)

Write chemical equations for the acid ionizations of each of the following weak acids (express these in terms of \(\left.\mathrm{H}_{3} \mathrm{O}^{+}\right)\) a. \(\mathrm{HCO}_{2} \mathrm{H}\) (formic acid) b. HF (hydrofluoric acid) c. \(\mathrm{HN}_{3}\) (hydrazoic acid) d. HOCN (cyanic acid)

Acrylic acid, whose formula is \(\mathrm{HC}_{3} \mathrm{H}_{3} \mathrm{O}_{2}\) or \(\mathrm{HO}_{2} \mathrm{CCH}=\mathrm{CH}_{2}\), is used in the manufacture of plastics. \(0.77 M\) aqueous solution of acrylic acid has a pH of 2.18 . What is \(K_{a}\) for acrylic acid?

Heavy metal azides, which are salts of hydrazoic acid, HN3, are used as explosive detonators. A solution of \(0.20 M\) hydrazoic acid has a pH of 3.21 . What is the \(K_{a}\) for hydrazoic acid?

Barbituric acid, \(\mathrm{HC}_{4} \mathrm{H}_{3} \mathrm{~N}_{2} \mathrm{O}_{3},\) is used to prepare various barbiturate drugs (used as sedatives). Calculate the concentrations of hydronium ion and barbiturate ion in a \(0.10 M\) solution of the acid. The value of \(K_{a}\) is \(9.8 \times 10^{-5}\).

A solution of acetic acid, \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2},\) on a laboratory shelf was of undetermined concentration. If the \(\mathrm{pH}\) of the solution was found to be 2.57 , what was the concentration of the acetic acid? The \(K_{a}\) of acetic acid is \(1.7 \times 10^{-5}\).

A chemist wanted to determine the concentration of a solution of lactic acid, \(\mathrm{HC}_{3} \mathrm{H}_{5} \mathrm{O}_{3} .\) She found that the \(\mathrm{pH}\) of the solution was 2.60 . What was the concentration of the solution? The \(K_{a}\) of lactic acid is \(1.4 \times 10^{-4}\).

Chloroacetic acid, \(\mathrm{HC}_{2} \mathrm{H}_{2} \mathrm{ClO}_{2},\) has a greater acid strength than acetic acid, because the electronegative chlorine atom pulls electrons away from the \(\mathrm{O}-\mathrm{H}\) bond and thus weakens it. Calculate the hydronium-ion concentration and the \(\mathrm{pH}\) of a \(0.0020 \mathrm{M}\) solution of chloroacetic acid. \(K\) is \(1.3 \times 10^{-3}\)

What is the hydronium-ion concentration of a \(2.00 M\) solution of 2,6 -dinitrobenzoic acid, \(\left(\mathrm{NO}_{2}\right)_{2} \mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COOH},\) for which \(K_{a}=7.94 \times 10^{-2} ?\)

What is the hydronium-ion concentration of a \(3.00 \times\) \(10^{-4} M\) solution of \(p\) -bromobenzoic acid, \(\mathrm{BrC}_{6} \mathrm{H}_{4} \mathrm{COOH},\) for which \(K_{a}=1.00 \times 10^{-4} ?\)

Phthalic acid, \(\mathrm{H}_{2} \mathrm{C}_{8} \mathrm{H}_{4} \mathrm{O}_{4},\) is a diprotic acid used in the synthesis of phenolphthalein indicator. \(K_{a 1}=1.2 \times\) \(10^{-3},\) and \(K_{a 2}=3.9 \times 10^{-6} .\) a Calculate the hydronium-ion concentration of a \(0.015 M\) solution. b What is the concentration of the \(\mathrm{C}_{8} \mathrm{H}_{4} \mathrm{O}_{4}{\underline{\phantom{xx}}}_{4}^{2-}\) ion in the solution?

Write the chemical equation for the base ionization of methylamine, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\). Write the \(K_{b}\) expression for methylamine.

Write the chemical equation for the base ionization of aniline, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} .\) Write the \(K_{b}\) expression for aniline.

Butylamine, \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NH}_{2}\) is a weak base. A \(0.47 M\) aqueous solution of butylamine has a pH of \(12.13 .\) What is \(K_{b}\) for butylamine? Calculate the \(\mathrm{pH}\) of a \(0.35 \mathrm{M}\) aqueous solution of butylamine.

Trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\) is a gas with a fishy, ammonialike odor. An aqueous solution that is \(0.25 M\) trimethylamine has a pH of \(11.63 .\) What is \(K_{b}\) for trimethylamine? Calculate the pH of a \(0.35 M\) aqueous solution of trimethylamine.

Write the equation for the acid ionization of the \(\mathrm{Zn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) ion.

Write the equation for the acid ionization of the \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) ion.

Obtain a the \(K_{b}\) value for \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{O}_{2}^{-} ;\) b the \(K_{a}\) value for \(\mathrm{NH}_{3} \mathrm{OH}^{+}\) (hydroxylammonium ion).

A buffer is prepared by mixing \(525 \mathrm{~mL}\) of \(0.50 \mathrm{M}\) formic acid, \(\mathrm{HCHO}_{2}\), and \(475 \mathrm{~mL}\) of \(0.50 \mathrm{M}\) sodium formate, \(\mathrm{NaCHO}_{2}\). Calculate the pH. What would be the pH of \(85 \mathrm{~mL}\) of the buffer to which \(8.6 \mathrm{~mL}\) of \(0.15 \mathrm{M}\) hydrochloric acid had been added?

What is the \(\mathrm{pH}\) of a buffer solution that is \(0.15 \mathrm{M}\) chloroacetic acid and \(0.10 \mathrm{M}\) sodium chloroacetate? \(K_{a}=\) \(1.3 \times 10^{-3}\)

What is the \(\mathrm{pH}\) of a solution in which \(15 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{NaOH}\) is added to \(25 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{HCl} ?\)

What is the \(\mathrm{pH}\) of a solution in which \(35 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{NaOH}\) is added to \(25 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{HCl} ?\)

A \(50.0-\mathrm{mL}\) sample of a \(0.100 \mathrm{M}\) solution of \(\mathrm{NaCN}\) is titrated by \(0.200 \mathrm{M} \mathrm{HCl} . K_{b}\) for \(\mathrm{CN}^{-}\) is \(2.0 \times 10^{-5}\). Calculate the \(\mathrm{pH}\) of the solution: a prior to the start of the titration; \(b\) after the addition of \(15.0 \mathrm{~mL}\) of \(0.200 \mathrm{M}\) \(\mathrm{HCl} ; \mathrm{c}\) at the equivalence point; d after the addition of \(30.0 \mathrm{~mL}\) of \(0.200 \mathrm{M} \mathrm{HCl}\)

Calculate the \(\mathrm{pH}\) of a solution obtained by mixing \(25.00 \mathrm{~mL}\) of \(0.19 \mathrm{M} \mathrm{NH}_{3}\) with \(25.00 \mathrm{~mL}\) of \(0.060 \mathrm{M} \mathrm{HCl}\)

Salicylic acid, \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{OHCOOH}\), is used in the manufacture of acetylsalicylic acid (aspirin) and methyl salicylate (wintergreen flavor). A saturated solution of salicylic acid contains \(2.2 \mathrm{~g}\) of the acid per liter of solution and has a pH of \(2.43 .\) What is the value of \(K_{a} ?\)

Cyanoacetic acid, \(\mathrm{CH}_{2} \mathrm{CNCOOH},\) is used in the manufacture of barbiturate drugs. An aqueous solution containing \(5.0 \mathrm{~g}\) in a liter of solution has a pH of 1.89 . What is the value of \(K_{a} ?\)

A \(0.050 M\) aqueous solution of sodium hydrogen sulfate, \(\mathrm{NaHSO}_{4},\) has a pH of 1.73. Calculate \(K_{a 2}\) for sulfuric acid. Sulfuric acid is a strong acid, so you can ignore hydrolysis of the \(\mathrm{HSO}_{4}^{-}\) ion.

Calculate the \(\mathrm{pH}\) of a \(0.072 \mathrm{M}\) aqueous solution of aluminum chloride, \(\mathrm{AlCl}_{3}\). The acid ionization of hydrated aluminum ion is $$ \mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{\underline{\phantom{xx}}}^{3+}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons $$ and \(K_{a}\) is \(1.4 \times 10^{-5}\).

An artificial fruit beverage contains \(11.0 \mathrm{~g}\) of tartaric acid, \(\mathrm{H}_{2} \mathrm{C}_{4} \mathrm{H}_{4} \mathrm{O}_{6},\) and \(20.0 \mathrm{~g}\) of its salt, potassium hydrogen tartrate, per liter. What is the \(\mathrm{pH}\) of the beverage? \(K_{a 1}=1.0 \times 10^{-3}\)