Chapter 14

One method for determining the purity of aspirin \(\left(\mathrm{C}_{9} \mathrm{H}_{8} \mathrm{O}_{4}\right)\) is to hydrolyze it with NaOH solution and then to titrate the remaining NaOH. The reaction of aspirin with NaOH is as follows: $$\begin{aligned} &\mathrm{C}_{9} \mathrm{H}_{8} \mathrm{O}_{4}(s)+2 \mathrm{OH}^{-}(a q)\\\&\text { Aspirin } \quad \frac{\text { Boil }}{10 \min } \underset{\text { Salicylate ion }}{\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}^{-}(a q)}+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \end{aligned}$$ A sample of aspirin with a mass of 1.427 g was boiled in \(50.00 \mathrm{mL}\) of \(0.500 \mathrm{M} \mathrm{NaOH} .\) After the solution was cooled, it took \(31.92 \mathrm{mL}\) of \(0.289 \mathrm{M}\) HCl to titrate the excess NaOH. Calculate the purity of the aspirin. What indicator should be used for this titration? Why?

A student intends to titrate a solution of a weak monoprotic acid with a sodium hydroxide solution but reverses the two solutions and places the weak acid solution in the buret. After \(23.75 \mathrm{mL}\) of the weak acid solution has been added to \(50.0 \mathrm{mL}\) of the \(0.100 \mathrm{M} \mathrm{NaOH}\) solution, the \(\mathrm{pH}\) of the resulting solution is \(10.50 .\) Calculate the original concentration of the solution of weak acid.

A sample of a certain monoprotic weak acid was dissolved in water and titrated with 0.125 \(M\) NaOH, requiring \(16.00 \mathrm{mL}\) to reach the equivalence point. During the titration, the pH after adding \(2.00 \mathrm{mL}\) NaOH was \(6.912 .\) Calculate \(K_{\mathrm{a}}\) for the weak acid.

Consider \(1.0 \mathrm{L}\) of a solution that is \(0.85 \mathrm{M} \mathrm{HOC}_{6} \mathrm{H}_{5}\) and \(0.80 M \mathrm{NaOC}_{6} \mathrm{H}_{5} .\left(K_{\mathrm{a}} \text { for } \mathrm{HOC}_{6} \mathrm{H}_{5}=1.6 \times 10^{-10} .\right)\) a. Calculate the \(\mathrm{pH}\) of this solution. b. Calculate the \(\mathrm{pH}\) after 0.10 mole of HCl has been added to the original solution. Assume no volume change on addition of HCl. c. Calculate the \(\mathrm{pH}\) after 0.20 mole of \(\mathrm{NaOH}\) has been added to the original buffer solution. Assume no volume change on addition of NaOH.

What concentration of \(\mathrm{NH}_{4} \mathrm{Cl}\) is necessary to buffer a \(0.52-M\) \(\mathrm{NH}_{3}\) solution at \(\mathrm{pH}=9.00 ?\left(K_{\mathrm{b}} \text { for } \mathrm{NH}_{3}=1.8 \times 10^{-5} .\right)\)

Consider the following acids and bases: Choose substances from the following list that would be the best choice to prepare a \(\mathrm{pH}=9.0\) buffer solution. a. \(\mathrm{HCO}_{2} \mathrm{H}\) b. HOBr c. \(\mathrm{KHCO}_{2}\) d. \(\mathrm{HONH}_{3} \mathrm{NO}_{3}\) \(\mathbf{e} .\left(\mathbf{C}_{2} \mathbf{H}_{5}\right)_{2} \mathrm{NH}\) f. \(\left(C_{2} H_{5}\right)_{2} N H_{2} C l\) g. \(\mathrm{HONH}_{2}\) h. NaOBr

Consider the titration of \(150.0 \mathrm{mL}\) of \(0.100 \mathrm{M}\) HI by \(0.250 \mathrm{M}\) NaOH. a. Calculate the \(\mathrm{pH}\) after \(20.0 \mathrm{mL}\) of \(\mathrm{NaOH}\) has been added. b. What volume of NaOH must be added so that the \(\mathrm{pH}=\) \(7.00 ?\)

Consider the titration of \(100.0 \mathrm{mL}\) of \(0.100 \mathrm{M}\) HCN by \(0.100 M \mathrm{KOH}\) at \(25^{\circ} \mathrm{C} .\left(K_{\mathrm{a}} \text { for } \mathrm{HCN}=6.2 \times 10^{-10} .\right)\) a. Calculate the \(\mathrm{pH}\) after \(0.0 \mathrm{mL}\) of \(\mathrm{KOH}\) has been added. b. Calculate the \(\mathrm{pH}\) after \(50.0 \mathrm{mL}\) of \(\mathrm{KOH}\) has been added. c. Calculate the \(\mathrm{pH}\) after \(75.0 \mathrm{mL}\) of \(\mathrm{KOH}\) has been added. d. Calculate the \(\mathrm{pH}\) at the equivalence point. e. Calculate the pH after 125 mL of KOH has been added.

Consider the titration of \(100.0 \mathrm{mL}\) of \(0.200 \mathrm{M}\) HONH \(_{2}\) by \(0.100 \mathrm{M}\) HCl. \(\left(K_{\mathrm{b}} \text { for } \mathrm{HONH}_{2}=1.1 \times 10^{-8} .\right)\) a. Calculate the \(\mathrm{pH}\) after \(0.0 \mathrm{mL}\) of HCI has been added. b. Calculate the \(\mathrm{pH}\) after \(25.0 \mathrm{mL}\) of HCl has been added. c. Calculate the \(\mathrm{pH}\) after \(70.0 \mathrm{mL}\) of HCl has been added. d. Calculate the \(\mathrm{pH}\) at the equivalence point. e. Calculate the \(\mathrm{pH}\) after \(300.0 \mathrm{mL}\) of HCl has been added. f. At what volume of HCl added does the \(\mathrm{pH}=6.04 ?\)

Consider the following four titrations (i-iv): i. \(150 \mathrm{mL}\) of \(0.2 \mathrm{M} \mathrm{NH}_{3}\left(K_{\mathrm{b}}=1.8 \times 10^{-5}\right)\) by \(0.2 \mathrm{M} \mathrm{HCl}\) ii. \(150 \mathrm{mL}\) of \(0.2 \mathrm{M}\) HCl by \(0.2 \mathrm{M} \mathrm{NaOH}\) iii. \(150 \mathrm{mL}\) of \(0.2 \mathrm{M} \mathrm{HOCl}\left(K_{\mathrm{a}}=3.5 \times 10^{-8}\right)\) by \(0.2 \mathrm{M} \mathrm{NaOH}\) iv. \(150 \mathrm{mL}\) of \(0.2 \mathrm{M} \mathrm{HF}\left(K_{\mathrm{a}}=7.2 \times 10^{-4}\right)\) by \(0.2 \mathrm{M} \mathrm{NaOH}\) a. Rank the four titrations in order of increasing \(\mathrm{pH}\) at the halfway point to equivalence (lowest to highest \(\mathrm{pH}\) ). b. Rank the four titrations in order of increasing \(\mathrm{pH}\) at the equivalence point. c. Which titration requires the largest volume of titrant (HCl or \(\mathrm{NaOH}\) ) to reach the equivalence point?

A buffer is made using \(45.0 \mathrm{mL}\) of \(0.750 \mathrm{M} \mathrm{HC}_{3} \mathrm{H}_{5} \mathrm{O}_{2}\left(K_{\mathrm{a}}=\right.\) \(1.3 \times 10^{-5}\) ) and \(55.0 \mathrm{mL}\) of \(0.700 \mathrm{M} \mathrm{NaC}_{3} \mathrm{H}_{5} \mathrm{O}_{2} .\) What volume of 0.10 \(M\) NaOH must be added to change the pH of the original buffer solution by \(2.5 \% ?\)

A \(0.400-M\) solution of ammonia was titrated with hydrochloric acid to the equivalence point, where the total volume was 1.50 times the original volume. At what \(\mathrm{pH}\) does the equivalence point occur?

What volume of \(0.0100 \mathrm{M}\) NaOH must be added to \(1.00 \mathrm{L}\) of \(0.0500 \mathrm{M}\) HOCI to achieve a pH of \(8.00 ?\)

Consider a solution formed by mixing \(50.0 \mathrm{mL}\) of \(0.100 \mathrm{M}\) \(\mathrm{H}_{2} \mathrm{SO}_{4}, 30.0 \mathrm{mL}\) of \(0.100 \mathrm{M} \mathrm{HOCl}, 25.0 \mathrm{mL}\) of \(0.200 \mathrm{M} \mathrm{NaOH}\), \(25.0 \mathrm{mL}\) of \(0.100 M \mathrm{Ba}(\mathrm{OH})_{2},\) and \(10.0 \mathrm{mL}\) of \(0.150 \mathrm{M} \mathrm{KOH}\) Calculate the pH of this solution.

When a diprotic acid, \(\mathrm{H}_{2} \mathrm{A}\), is titrated with \(\mathrm{NaOH}\), the protons on the diprotic acid are generally removed one at a time, resulting in a pH curve that has the following generic shape: a. Notice that the plot has essentially two titration curves. If the first equivalence point occurs at \(100.0 \mathrm{mL}\) NaOH added, what volume of NaOH added corresponds to the second equivalence point? b. For the following volumes of NaOH added, list the major species present after the OH \(^{-}\) reacts completely. i. \(0 \mathrm{mL} \mathrm{NaOH}\) added ii. between 0 and \(100.0 \mathrm{mL}\) NaOH added iii. \(100.0 \mathrm{mL}\) NaOH added iv. between 100.0 and \(200.0 \mathrm{mL}\) NaOH added v. \(200.0 \mathrm{mL} \mathrm{NaOH}\) added vi. after \(200.0 \mathrm{mL}\) NaOH added c. If the \(\mathrm{pH}\) at \(50.0 \mathrm{mL}\) NaOH added is \(4.0,\) and the \(\mathrm{pH}\) at \(150.0 \mathrm{mL} \mathrm{NaOH}\) added is \(8.0,\) determine the values \(K_{\mathrm{a}_{1}}\) and \(K_{\mathrm{a}_{2}}\) for the diprotic acid.

Malonic acid \(\left(\mathrm{HO}_{2} \mathrm{CCH}_{2} \mathrm{CO}_{2} \mathrm{H}\right)\) is a diprotic acid. In the titration of malonic acid with NaOH, stoichiometric points occur at \(\mathrm{pH}=3.9\) and \(8.8 .\) A 25.00 -mL sample of malonic acid of unknown concentration is titrated with 0.0984 \(M \mathrm{NaOH},\) requiring \(31.50 \mathrm{mL}\) of the NaOH solution to reach the phenolphthalein end point. Calculate the concentration of the initial malonic acid solution. (See Exercise \(113 .\) )

A \(10.00-g\) sample of the ionic compound \(\mathrm{NaA}\), where \(\mathrm{A}^{-}\) is the anion of a weak acid, was dissolved in enough water to make 100.0 mL of solution and was then titrated with 0.100 \(M\) HCl. After 500.0 mL HCl was added, the pH was \(5.00 .\) The experimenter found that 1.00 L of \(0.100 M\) HCl was required to reach the stoichiometric point of the titration. a. What is the molar mass of NaA? b. Calculate the \(p\) H of the solution at the stoichiometric point of the titration.