Question 1

Which of the following best describes the common ion effect, as illustrated by the addition of concentrated HCl to a saturated NaCl solution?

Accepted Answer

The addition of a common ion decreases the solubility of NaCl by shifting equilibrium to the left.

Question 2

A buffer solution is prepared using 0.50 M acetic acid ($$\mathrm{HC_2H_3O_2}$$, $$K_a = 1.8 	imes 10$$^{-5}$$) and 0.50 M sodium acetate ($$\mathrm{NaC_2H_3O_2}$$). What is the pH of this buffer? (Use pK_a$$ = -\log(1.8 	imes $$10^{-5}$$)$$)

Accepted Answer

$$pH = 4.74 + \log\left(\frac{0.50}{0.50}\right) = 4.74$$

Question 3

Which of the following statements about buffer capacity is correct?

Accepted Answer

Buffer capacity is maximized when the concentrations of acid and conjugate base are both large and equal.

Question 4

A chemist needs a buffer at pH 4.30. Which acid from the following list is the best choice? (a) Chloroacetic acid, $$K_a = 1.35 	imes 10$$^{-3}$$; (b) Propanoic acid, K_a = 1.3$$ 	imes $$10^{-5}$$; (c) Benzoic acid, $$K_a = 6.4 	imes 10$$^{-5}$$; (d) Hypochlorous acid, K_a = 3.5$$ 	imes $$10^{-8}.$$

Accepted Answer

Propanoic acid

Question 5

Refer to the titration curve for the titration of 25.00 mL of 0.100 M acetic acid with 0.100 M NaOH. At the halfway point to the equivalence, what is the pH?

Accepted Answer

The pH equals the $$pK_a of $$acetic acid.

Question 6

Which of the following best explains why the pH of pure water jumps from 7.00 to 12.00 when 0.01 mol NaOH is added to 1.0 L of water, as shown in the images?

Accepted Answer

Pure water has no buffer capacity, so the addition of a small amount of strong base causes a large pH change.

Question 7

A solution contains 0.25 M NH$$_3$$ ($$K_b = 1.8 	imes 10$$^{-5}$$) and 0.40 M NH_4$Cl. What is the pH of this buffer? (pK_b$$ = -\log(1.8 	imes $$10^{-5}$$)$$; pK_a + pK_b = 14$)

Accepted Answer

$$pH = 9.26 + \log\left(\frac{0.40}{0.25}\right) = 9.87$$

Question 8

Which of the following best describes the effect of adding a strong acid to a buffer solution containing acetic acid and sodium acetate?

Accepted Answer

The added H$$^+$$ reacts with C$$_2H$_3O$_2^- to $$form more HC$$_2H$_3O$_2$$, minimizing the pH change.

Question 9

Which of the following is the correct $$K_{sp}$$ expression for the dissolution of $$\mathrm{$$CaF_2$$}$$?

Accepted Answer

$$K_{sp} = [Ca$$^{2+}$$][F^-]^2$$

Question 10

A solution is prepared by mixing 150.0 mL of $$1.00 	imes 10$$^{-2}$$ M Mg(NO_3$)_2$ and 250.0 mL of 1.00$$ 	imes $$10^{-1} M $$NaF. What is the equilibrium concentration of Mg$$^{2+}$$ with solid MgF$$_2$$ ($$K_{sp} = 6.4 	imes 10$$^{-9}$$)?

Accepted Answer

$$[Mg$$^{2+}$$] = \frac{K_{sp}}{[F^-]^2}$$

Question 11

Which of the following best explains why magnesium hydroxide is more soluble in acidic solution than in basic solution?

Accepted Answer

Acid reacts with OH$$^-$$, shifting the equilibrium to dissolve more Mg(OH)$$_2.$$

Question 12

Which of the following is true regarding the precipitation of PbI$$_2$$ ($$K_{sp} = 1.4 	imes 10$$^{-8}$$) and CuI (K$$_{sp} = 5.3 	imes $$10^{-12}) $$from a solution containing $$1.0 	imes 10$$^{-4}$$ M Cu$$^+$$ and 2.0$$ 	imes $$10^{-3} M Pb$^{2+} as I$^- is $$added?

Accepted Answer

CuI will precipitate first because it requires a lower [I$$^-$$] to reach $$K_{sp}.$$

Question 13

Which of the following best describes the role of a complex ion in increasing the solubility of an otherwise insoluble salt, such as AgCl in ammonia?

Accepted Answer

The formation of [Ag(NH$$_3$$)$$_2$$]$$^+$$ removes Ag$$^+$$ from solution, shifting the dissolution equilibrium of AgCl to the right.

Question 14

Which of the following is the correct expression for the overall equilibrium constant for dissolving AgCl in ammonia, given $$K_{sp}$$ for AgCl and $$K_f$$ for [Ag(NH$$_3$$)$$_2$$]$$^+$$?

Accepted Answer

$$K = K_{sp} 	imes K_f$$

Question 15

Which of the following best explains why salts containing anions that are conjugate bases of weak acids (e.g., carbonates, sulfides) are more soluble in strong acids?

Accepted Answer

The acid reacts with the anion, shifting the dissolution equilibrium to the right.

Question 16

Which of the following is the correct pH range for the color change of phenolphthalein, as shown in the indicator chart?

Accepted Answer

pH 8–10

Question 17

A titration curve for a weak acid with a strong base shows a gradual increase in pH, followed by a sharp rise near the equivalence point. What does the pH at the equivalence point depend on?

Accepted Answer

The $$K_a of $$the weak acid

Applications of Aqueous Equilibria: Acids, Bases, Buffers, Titrations, and Solubility

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