Chapter 15

How does a buffer "kill" added strong base?

How does a buffer "kill" added strong acid?

Write the equations that show how a hypochlorous acid buffer defends against added strong acid and base.

Perchloric acid, \(\mathrm{HClO}_{4}\), is a strong acid. (a) Write the chemical equation for the reaction between perchloric acid and water. (b) List all species present in an aqueous solution of perchloric acid in order of concentration, highest to lowest. (c) Calculate the concentration of all species (except \(\mathrm{H}_{2} \mathrm{O}\) ) present in a \(0.100 \mathrm{M} \mathrm{HClO}_{4}\) solution. (d) What is the \(\mathrm{pH}\) of this solution?

Pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\), is a weak base. (a) Write the chemical equation for the reaction between pyridine and water. (b) List all species present in an aqueous solution of pyridine in order of concentration, highest to lowest. (c) In a 0.100 M pyridine solution, \(3.2 \%\) of the pyridine has reacted with water to form products. Calculate the concentration of all species present (except water) in \(1 \mathrm{~L}\) of a \(0.100\) M pyridine solution. (d) What is the pH of this solution?

In each of the following pairs, which is the stronger acid? (a) \(\mathrm{HPO}_{4}^{2-}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{HCN}\left(K_{\mathrm{eq}}=6.2 \times 10^{-10}\right)\) and \(\mathrm{HCO}_{2} \mathrm{H}\left(K_{\mathrm{eq}}=1.8 \times 10^{-4}\right)\) (d) HI and HF

A solution is formed by mixing \(50.0 \mathrm{~mL}\) of \(0.015 \mathrm{M} \mathrm{NaOH}(a q)\) with \(50.0 \mathrm{~mL}\) of \(0.010 \mathrm{M}\) \(\mathrm{HNO}_{3}(a q)\) (a) Is the solution acidic or basic? (b) Draw a beaker and show all species present in the solution. (c) What is the pH of the solution?

If \(100 \mathrm{~mL}\) of an aqueous solution of nitric acid contains \(0.030\) mole of \(\mathrm{HNO}_{3}\), what is the \(\mathrm{pH}\) of the solution?

Write a chemical equation for the reaction between each pair of reactants, using single or double arrows as appropriate: (a) \(\mathrm{HNO}_{3}\) and \(\mathrm{OH}^{-}\) (b) \(\mathrm{HF}\) and \(\mathrm{OH}^{-}\) (c) \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}\)

A \(0.20 \mathrm{M}\) aqueous solution of monoprotic acid HX has a pH of \(2.14\). (a) Is HX a strong acid or a weak acid? (b) Calculate \(K_{\text {eq }}\) for the reaction of HX with water. (Hint: What are the equilibrium concentrations of \(\mathrm{H}_{3} \mathrm{O}^{+}, \mathrm{X}^{-}\), and HX?)

Without calculating \(\mathrm{pH}\) values, list these solutions in order of acidity, from lowest \(\mathrm{pH}\) to highest \(\mathrm{pH}\) : (a) \(0.10 \mathrm{M} \mathrm{LiNO}_{3}\) (b) \(0.10 \mathrm{M} \mathrm{NaF}\) (c) \(0.10 \mathrm{M} \mathrm{KOH}\) (d) \(0.10 \mathrm{M} \mathrm{HCN}\) (e) \(0.10 \mathrm{M} \mathrm{HNO}_{3}\) (f) \(0.20 \mathrm{M} \mathrm{HCl}\)

How many moles of nitric acid do you need to prepare \(200 \mathrm{~mL}\) of an aqueous solution that has a pH of \(2.0\) ?

Other than water, what would you expect to find in the highest concentration in an aqueous solution of \(\mathrm{Ca}(\mathrm{OH})_{2} ?\) Explain.

Write a chemical equation for the reaction between each pair of reactants, using single or double arrows as appropriate: (a) \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{OH}^{-}\) (b) \(\mathrm{HCl}\) and \(\mathrm{F}^{-}\) (c) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) and \(\mathrm{OH}^{-}\) (d) \(\mathrm{HCN}\) and \(\mathrm{H}_{2} \mathrm{O}\)

Other than water, what would you expect to find in the highest concentration in an aqueous solution of KOH? Explain.

What is the \(\mathrm{pH}\) of these aqueous solutions? (a) \(1.0 \mathrm{M} \mathrm{HCl}\), (b) \(0.1 \mathrm{M} \mathrm{HCl}\), (c) \(0.001 \mathrm{M} \mathrm{HCl}\), (d) \(1.0 \times 10^{-5} \mathrm{M} \mathrm{HCl}\), (e) \(1.10 \times 10^{-7} \mathrm{M} \mathrm{HCl}\)

How many moles of \(\mathrm{Ca}(\mathrm{OH})_{2}\) would it take to completely neutralize \(0.4\) mole of phosphoric acid, \(\mathrm{H}_{3} \mathrm{PO}_{4} ?\)

Sulfuric acid, \(\mathrm{H}_{2} \mathrm{SO}_{4}\), is a diprotic acid with dissociation equilibrium constants of \(K_{\mathrm{eq}}>1.0 \times 10^{3}\) and \(K_{\mathrm{eq}}=1.2 \times 10^{-2} .\) Write the two dissociation equilibrium equations, and match the proper \(K_{\text {eq }}\) to each. Which species is the weak acid?

What is the pH of a \(0.010 \mathrm{M}\) aqueous solution of \(\mathrm{NaOH}\) ?

Which acid and base react to give an aqueous solution of \(\mathrm{K}_{2} \mathrm{SO}_{4} ?\) Write a balanced equation for this neutralization reaction.

The molar concentration of concentrated hydrochloric acid is \(12.1 \mathrm{M}\). What are the \(\mathrm{pH}\) and \(\mathrm{OH}^{-}\) ion molar concentration?

Rewrite this acid-base reaction in dot-diagram form, and then use an arrow to show the proton being transferred and label the Brønsted-Lowry acid and base: \(\mathrm{F}^{-}+\mathrm{NH}_{4}^{+} \rightleftarrows \mathrm{HF}+\mathrm{NH}_{3}\)

Indicate whether each compound is an electrolyte or a nonelectrolyte in water: (a) \(\mathrm{N}_{2}\) (b) \(\mathrm{NH}_{4} \mathrm{ClO}_{4}\) (c) \(\mathrm{HI}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (propane) (e) \(\mathrm{Li}_{3} \mathrm{PO}_{4}\) (f) \(\mathrm{AgNO}_{3}\) (g) \(\mathrm{CCl}_{4}\)

Hydrazine, \(\mathrm{H}_{2} \mathrm{NNH}_{2}\), is a weak base that can accept two protons. Draw a dot diagram for hydrazine and write equations showing how this compound accepts protons from water in two steps.

Acrylic acid, \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{COOH}\), is a weak monoprotic acid. Write a balanced equation for the reaction that occurs when \(25.0 \mathrm{~mL}\) of \(0.200 \mathrm{M} \mathrm{KOH}\) is added to \(50.0 \mathrm{~mL}\) of \(0.200 \mathrm{M} \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{COOH}\). Besides water, what species are present in the solution? Is this solution a buffer? Why or why not?

How many grams of \(\mathrm{LiOH}\) are there in \(750 \mathrm{~mL}\) of an aqueous LiOH solution having an \(\mathrm{H}_{3} \mathrm{O}^{+}\) concentration of \(2.30 \times 10^{-13} \mathrm{M} ?\)

Citric acid, \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}\), is a weak triprotic acid. Write equations for the three equilibrium reactions that occur during the stepwise dissociation of citric acid in water.

What is the molar hydroxide ion concentration in a solution that is 1000 times more acidic than a solution that has a pH of \(9.20\) ?

Using dot diagrams, write a reaction showing how the phenoxide ion, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}^{-}\) can act as a weak base in water.

For each acid-base pair, write a balanced equation for the neutralization reaction: (a) Lithium hydroxide and hydroiodic acid (b) Acetic acid and sodium hydroxide (c) Hydrobromic acid and calcium hydroxide (d) Potassium hydroxide and phosphoric acid (assume all acidic protons react)

Is a solution that contains \(0.10 \mathrm{M} \mathrm{HBr}\) and \(0.10 \mathrm{M}\) NaBr a buffered solution? Why or why not?

Formic acid, \(\mathrm{HCHO}_{2}\), has a \(K_{\mathrm{eq}}\) of \(1.8 \times 10^{-4}\), and propionic acid, \(\mathrm{HC}_{3} \mathrm{H}_{5} \mathrm{O}_{2}\), has a \(K_{\mathrm{eq}}\) of \(1.3 \times 10^{-5}\) Which has a higher \(\mathrm{pH}\), a \(1 \mathrm{M}\) solution of formic acid or a \(1 \mathrm{M}\) solution of propionic acid?

How many grams of \(\mathrm{HCl}\) gas are dissolved in \(7.50 \mathrm{~L}\) of an aqueous \(\mathrm{HCl}\) solution that has a \(\mathrm{pH}\) of \(2.40 ?\)

Classify each substance as strong electrolyte, weak electrolyte, or nonelectrolyte: (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{KCH}_{3} \mathrm{COO}\) (c) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (e) \(\mathrm{NH}_{3}\) (f) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (g) \(\mathrm{ZnSO}_{4}\)

\(\mathrm{HClO}_{4}\) is a strong acid, and \(\mathrm{HClO}_{2}\) is a weak acid. If you had a \(1.0 \mathrm{M}\) solution of \(\mathrm{NaClO}_{4}\) and a \(1.0 \mathrm{M}\) solution of \(\mathrm{NaClO}_{2}\), which would have the higher pH? Explain.

If you dissolve \(0.250 \mathrm{~g}\) of \(\mathrm{Ba}(\mathrm{OH})_{2}\) in \(3.00 \mathrm{~L}\) of water, what is the \(\mathrm{pH}\) of the solution?

An aqueous solution containing \(\mathrm{CN}^{-}\) ions turns litmus blue. Write an equation showing how \(\mathrm{CN}^{-}\) makes water basic.

You mix \(500 \mathrm{~mL}\) of \(1.00 \mathrm{M} \mathrm{NaOCl}\) with \(500 \mathrm{~mL}\) of \(0.500 \mathrm{MHNO}_{3}\). Write an equation for the reaction that occurs. Besides water, what species are in the solution after reaction? Is this solution a buffer?

If \(0.378 \mathrm{~g}\) of HBr dissolved in enough water to make \(1.25 \mathrm{~L}\) of solution, what is the \(\mathrm{H}_{3} \mathrm{O}^{+}\) concentration? What is the \(\mathrm{OH}^{-}\) concentration?

Rewrite this acid-base reaction in dot-diagram form, and then use an arrow to show the proton being transferred and label the Brønsted-Lowry acid and base: \(\mathrm{HS}^{-}+\mathrm{HOCl} \rightleftarrows \mathrm{H}_{2} \mathrm{~S}+\mathrm{OCl}^{-}\)

What is the \(\mathrm{OH}^{-}\) concentration of a solution that has a pH of \(9.66\) ? Is this solution acidic or basic?

An ionic compound with the formula \(\operatorname{NaX}(\mathrm{X}\) is an unknown anion) is dissolved in water, and the resulting solution is basic. Is HX a strong acid or a weak acid? Explain.

Lithium hydride (LiH) is a source of hydride ion, shown below. \(\mathrm{H}\) : As you can see, the hydride ion has a negative charge (it is an anion) and a lone pair of electrons. When thrown into water, the hydride ion makes the water strongly basic. Write a reaction that shows how the hydride ion reacts with water to make the water basic. Then explain in English what the hydride ion does to water molecules to make the resulting solution basic.

When \(\mathrm{Fe}^{3+}\) ions dissolve in water, they immediately form \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{3+}\) aqueous ions in which six molecules of water bind to the iron via six Fe-O bonds. However, this is not all that happens. The solution also becomes acidic. Interestingly, \(\mathrm{Fe}^{2+}\) ions also form similar \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}\) ions, but the solution does not become acidic. Explain why one solution becomes acidic and the other does not. (Hint: The more positive the metal center, the more it attracts electrons to itself. Think about the effect this has on bonds within the ions.)

A quantity often used to characterize the acidity of an acid is its \(\mathrm{p} K_{\mathrm{a}}\). The \(\mathrm{pK}_{\mathrm{a}}\) of an acid is equal to \(-\log \left(K_{\mathrm{a}}\right)\). Knowing this, what can you say about acid #1 whose \(\mathrm{pK}_{\mathrm{a}}=2\), and acid \(\\# 2\) whose \(\mathrm{p} K_{\mathrm{a}}=6 ?\) Which is more acidic and by how much? (Hint: if \(\mathrm{pK}_{\mathrm{a}}=-\log \left(K_{\mathrm{a}}\right)\), then \(\left.K_{\mathrm{a}}=10^{-\mathrm{pKa}} .\right)\)

Complete the following sentence: "The lower the \(\mathrm{p} K_{\mathrm{a}}\), the (stronger/weaker) the acid." \(^{\prime \prime}\) How does this parallel pH?

Glycine is the simplest of amino acids and is shown below. the amino acid glycine Notice that it has a weak COOH end similar to acetic acid. Like acetic acid, this end is weakly acidic. It also has an \(\mathrm{NH}_{2}\) end similar to \(\mathrm{NH}_{3}\). Like \(\mathrm{NH}_{3}\), this end is weakly basic. Although not technically a buffer, when glycine is added to water, the solution is buffered against addition of strong acid or base. Explain why.

Actually, once in water, the amino acid glycine (see problem above) exists as a zwitterion, as shown below. A zwitterion is an overall electrically neutral species that nevertheless has \(\mathrm{a}+\) and \(-\) charge in it. Zwitterionic form of glycine Now, the \(\mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+}\) end is weakly acidic and the \(\mathrm{COO}^{-}\) end is weakly basic. Does this mean that the zwitterionic form cannot supply buffering action against strong acid or strong base in water? Explain.

What is the \(\mathrm{pH}\) of a solution prepared by mixing \(100.00 \mathrm{~mL}\) of \(0.020 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) with \(50.00 \mathrm{~mL}\) of \(0.100\) M \(\mathrm{NaOH}\) ? Assume that the volumes are additive.

Determine the ammonia concentration of an aqueous solution that has a pH of \(11.50\). The equation for the dissociation of \(\mathrm{NH}_{3}\) \(\left(\mathrm{K}_{\mathrm{b}}=1.8 \times 10^{-5}\right)\) is \(\mathrm{NH}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows\) \(\mathrm{NH}_{4}^{+}(a q)+\mathrm{OH}^{-}(a q)\)