Chapter 18

Explain the difference between a strong acid and a weak acid.

Explain why equilibrium arrows are used in the ionization equations for some acids.

How would you compare the strengths of two weak acids experimentally? By looking up information in a table or a handbook?

Identify the conjugate acid-base pairs in the reaction of \(\mathrm{H}_{3} \mathrm{PO}_{4}\) with water.

Ammonia Cleaner Write the chemical equation and \(K_{\mathrm{b}}\) expression for the ionization of ammonia in water. How is it safe for a window cleaner to use a solution of ammonia, which is basic?

Disinfectant Hypochlorous acid is an industrial disinfectant. Write the chemical equation and the \(K_{\text {a }}\) expression for the ionization of hypochlorous acid in water.

A fictional weak base, \(\mathrm{ZaH}_{2}\) , reacts with water to yield a solution with a \(\mathrm{OH}^{-}\) ion concentration of \(2.68 \times 10^{-4}\) \(\mathrm{mol} / \mathrm{L}\) . The chemical equation for the reaction is \(\mathrm{ZaH}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1) \rightleftharpoons \mathrm{ZaH}_{3}+(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) .\) If \(\left[\mathrm{ZaH}_{2}\right]\) at equilibrium is \(0.0997 \mathrm{mol} / \mathrm{L},\) what is the value of \(K_{\mathrm{b}}\) for \(\mathrm{ZaH}_{2}\) ?

Select a strong acid, and explain how you would prepare a dilute solution of the acid. Select a weak acid, and explain how you would prepare a concentrated solution of the acid.

What is the relationship between the pOH and the OH- ion concentration of a solution?

Solution A has a pH of 2.0. Solution B has a pH of 5.0. Which solution is more acidic? Based on the H+ ion concentrations in the two solutions, how many times more acidic?

If the concentration of \(\mathrm{H}^{+}\) ions in an aqueous solution decreases, what must happen to the concentration of \(\mathrm{OH}^{-}\) ions? Why?

Use Le Chatelier's principle to explain what happens to the equilibrium \(\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})\) when a few drops of HCl are added to pure water.

If 5.00 \(\mathrm{mL}\) of 6.00 \(\mathrm{M} \mathrm{HCl}\) is added to 95.00 \(\mathrm{mL}\) of pure water, the final volume of the solution is 100.00 \(\mathrm{mL}\) . What is the \(\mathrm{pH}\) of the solution?

Given two solutions, 0.10\(M \mathrm{HCl}\) and 0.10 \(\mathrm{M}\) HF, which solution has the greater concentration of \(\mathrm{H}^{+}\) ions? Calculate \(\mathrm{pH}\) values for the two solutions, given that \(\left[\mathrm{H}^{+}\right]=7.9 \times 10^{-3} \mathrm{M}\) in the 0.10 \({M}$$\mathrm{HF}\).

Metal Cleaner Chromic acid is used as an industrial cleaner for metals. What is \(K_{\mathrm{a}}\) for the second ionization of chromic acid \(\left(\mathrm{H}_{2} \mathrm{CrO}_{4}\right)\) if a 0.040\(M\) solution of sodium hydrogen chromate has a pH of 3.946\(?\)

When might a pH meter be better than an indicator to determine the end point of an acid-base titration?

What happens when an acid is added to a solution containing the HF/F- buffer system?

Write formula equations and net ionic equations for the hydrolysis of each salt in water. \begin{equation} \text { a. sodium carbonate }\quad\text { b. ammonium bromide } \end{equation}

Air Purifier Lithium hydroxide is used to purify air by removing carbon dioxide. A 25.00 -mL sample of lithium hydroxide solution is titrated to an end point by 15.22 mL of 0.3340 \(\mathrm{M}\) hydrochloric acid solution. What is the molarity of the LiOH solution?

In an acid-base titration, 45.78 \(\mathrm{mL}\) of a sulfuric acid solution is titrated to the end point by 74.30 \(\mathrm{mL}\) of 0.4388 \(\mathrm{M}\) sodium hydroxide solution. What is the molarity of the \(\mathrm{H}_{2} \mathrm{SO}_{4}\) solution?

Write the equation for the ionization reaction and the base ionization constant expression for ethylamine \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{N} \mathrm{H}_{2}\right)\) in water.

How many milliliters of 0.225\(M \mathrm{HCl}\) would be required to titrate 6.00 \(\mathrm{g}\) of \(\mathrm{KOH}\) ?

What is the \(\mathrm{pH}\) of a 0.200\(M\) solution of hypobromous acid \((\mathrm{HBrO}) ? K_{\mathrm{a}}=2.8 \times 10^{-9}\)

Which of the following are polyprotic acids? Write successive ionization equations for the polyprotic acids in water. \begin{equation} \begin{array}{ll}{\text { a. } \mathrm{H}_{3} \mathrm{BO}_{3}} & {\text { c. HNO }_{3}} \\ {\text { b. } \mathrm{CH}_{3} \mathrm{COOH}} & {\text { d. } \mathrm{H}_{2} \mathrm{SeO}_{3}}\end{array} \end{equation}

Write balanced chemical equations for the two successive ionizations of carbonic acid in water. Identify the conjugate-base pair in each of the equations.

Sugar Refining Strontium hydroxide is used in the refining of beet sugar. Only 4.1 g of strontium hydroxide can be dissolved in 1 L of water at 273 K. Given that its solubility is so low, explain how it is possible that strontium hydroxide is considered a strong base.

What are the concentrations of \(\mathrm{OH}^{-}\) ions in solutions having \(\mathrm{pH}\) values of \(3.00,6.00,9.00,\) and 12.00 at 298 \(\mathrm{K}\) ? What are the pOH values for the solutions?

Write the chemical equation for the reaction that would occur when a base is added to a solution containing the \(\mathrm{H}_{2} \mathrm{PO}_{4}-/ \mathrm{HPO}_{4}^{2-}\) buffer system.

An aqueous solution buffered by benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right.\) \(\mathrm{COOH}\) ) and sodium benzoate \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOHNa}\right)\) is 0.0500 \(\mathrm{M}\) in both compounds. Given that benzoic acid's \(K_{\mathrm{a}}\) equals \(6.4 \times 10^{-5},\) what is the pH of the solution?

Critique the following statement: "A substance whose chemical formula contains a hydroxyl group must be considered to be a base."

Analyze and Conclude Is it possible that an a Arrhenius acid is not a Bronsted-Lowry acid? Is it possible that an acid according to the Bronsted-Lowry modelis not an Arrhenius acid? Is it possible that a Lewis acid could not be classified as either an Arrhenius or a Bronsted-Lowry acid? Explain and give examples.

Apply Concepts Use the ion product constant of water at 298 \(\mathrm{K}\) to explain why a solution with a pH of 3.0 must have a poH of \(11.0 .\)

Identify the Lewis acids and bases in the following reactions. \begin{equation} \begin{array}{l}{\text { a. } \mathrm{H}^{+}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{O}} \\ {\text { b. } \mathrm{Cl}^{-}+\mathrm{BCl}_{3} \rightleftharpoons \mathrm{BCl}_{4}^{-}} \\\ {\text { c. } \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{4}}\end{array} \end{equation}

Interpret Scientific Illustrations Sketch the shape of the approximate pH v. volume curve that would result from titrating a diprotic acid with a 0.10M NaOH solution.

Recognize Cause and Effect Illustrate how a buffer works using the \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{3}+/ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) buffer system. Show with equations how the weak base/conjugate acid system is affected when small amounts of acid and base are added to a solution containing this buffer system.

Apply Concepts Like all equilibrium constants, the value of \(K_{w}\) varies with temperature. \(K_{\mathrm{w}}\) equals \(2.92 \times 10^{-15}\) at \(10^{\circ} \mathrm{C}, 1.00 \times 10^{-14}\) at \(25^{\circ} \mathrm{C},\) and 2.92 \(\mathrm{x}\) \(10^{-14}\) at \(40^{\circ} \mathrm{C} .\) In light of this information, calculate and compare the pH values for pure water at these three temperatures. Based on your calculations, is it correct to say that the pH of pure water is always 7.0? Explain

You have 20.0 \(\mathrm{mL}\) of a solution of a weak acid, \(\mathrm{HX}\) , whose \(K_{\mathrm{a}}\) equals \(2.14 \times 10^{-6} .\) The \(\mathrm{pH}\) of the solution is found to be \(3.800 .\) How much distilled water would you have to add to the solution to increase the pH to 4.000\(?\)

What factors determine whether a molecule is polar or nonpolar? (Chapter 8)

What property of some liquids accounts for the meniscus that forms at the surface of a solution in a buret? (Chapter 12)

Which of the following physical processes are exothermic for water-freezing, boiling, condensing, subliming, evaporating? (Chapter 12)

Explain why an air pump gets hot when you pump air into a bicycle tire. (Chapter 13)

When 5.00 g of a compound was burned in a calorimeter, the temperature of 2.00 \(\mathrm{kg}\) of water increased from \(24.5^{\circ} \mathrm{C}\) to \(40.5^{\circ} \mathrm{C}\) . How much heat would be released by the combustion of 1.00 mol of the compound (molar mass = 46.1 g/mol)? (Chapter 15)

What is the difference between an exothermic and an endothermic reaction? (Chapter 15)

Hydrogen and fluorine react to form HF according to the following equilibrium equation. $$\mathrm{H}_{2}(\mathrm{g})+\mathrm{F}_{2}(\mathrm{g}) \rightleftharpoons 2 \mathrm{HF} \Delta H=-538 \mathrm{kJ}(\mathrm{g})$$ Will raising the temperature cause the amount of product to increase? Explain. (Chapter 17)

Acid/Base Theories Imagine that you are the Danish chemist Johannes Bronsted. The year is 1923, and you have formulated a new theory of acids and bases. Write a letter to Swedish chemist Svante Arrhenius in which you discuss the differences between your theory and his and point out the advantages of yours.

Amino Acids Twenty amino acids combine to form proteins in living systems. Research the structures and \(K_{\text { a }}\)values for five amino acids. Compare the strengths of these acids with the acids in Table 18.4 .