Question 1

Which of the following best explains why the addition of sodium acetate to a solution of acetic acid decreases the concentration of hydrogen ions in the solution?

Accepted Answer

The common-ion effect shifts the equilibrium of acetic acid dissociation to the left, decreasing $$[H^+].$$

Question 2

A buffer is prepared by mixing 0.20 mol of lactic acid ($$pK_a = 3.85) $$and 0.10 mol of sodium lactate in 1.0 L of water. What is the pH of the buffer?

Accepted Answer

$$pH = 3.85 + \log \left( \frac{0.10}{0.20} \right) = 3.55$$

Question 3

Which of the following statements about buffer capacity is correct?

Accepted Answer

Buffer capacity is maximized when $$[HA] = [A^-]$$ and both concentrations are high.

Question 4

A 0.10 M solution of HF ($$K_a = 6.8 	imes 10$$^{-4}$$) is mixed with 0.10 M HNO_3$. What happens to the concentration of $$[HF]$$ and the pH?

Accepted Answer

$$[HF]$$ increases and pH decreases due to the common-ion effect from $$H^+.$$

Question 5

Which equation is used to calculate the pH of a buffer solution containing a weak acid and its conjugate base?

Accepted Answer

$$pH = pK_a + \log \left( \frac{[A^-]}{[HA]} \right)$$

Question 6

During the titration of 50.0 mL of 0.10 M acetic acid with 0.10 M NaOH, what is the pH at the equivalence point?

Accepted Answer

Greater than 7, because the acetate ion hydrolyzes to produce $$OH^-.$$

Question 7

Which of the following best describes the effect of adding NaF to a saturated solution of CaF$$_2$$?

Accepted Answer

The solubility of CaF$$_2$$ decreases due to the common-ion effect.

Question 8

A solution contains 0.10 M $$Ba^{2+}$$ and 0.10 M $$SO_4$$^{2-}$$. Given K$$_{sp}$$ for $$BaSO_4$$ is 1.1$$ 	imes $$10^{-10}$$, what will happen when these solutions are mixed?

Accepted Answer

A precipitate of $$BaSO_4$$ will form because $$Q \u003e K_{sp}.$$

Question 9

Which of the following is true regarding the solubility of $$Mg(OH)_2 in a $$solution buffered at pH 9.00 compared to pure water?

Accepted Answer

The solubility of $$Mg(OH)_2 is $$lower at pH 9.00 because $$[OH^-] is $$higher.

Question 10

Which of the following best explains why $$AgCl$$ becomes much more soluble in the presence of excess $$NH_3$$?

Accepted Answer

$$NH_3$$ forms a complex ion with $$Ag^+$$, reducing $$[Ag^+]$$ and shifting the dissolution equilibrium to the right.

Question 11

Which of the following salts is amphoteric and will dissolve in both acidic and basic solutions?

Accepted Answer

$$Al(OH)_3$$

Question 12

A solution contains $$1.0 	imes 10$$^{-2}$$ M Ag$$^+$$ and 2.0$$ 	imes $$10^{-2} M$$ $$Pb^{2+}$$. $$K_{sp}$$ for $$AgCl is$$ $$1.8 	imes 10$$^{-10}$$ and for $$PbCl_2$$ is 1.7$$ 	imes $$10^{-5}. $$What $$[Cl^-] is $$required to begin precipitation of $$AgCl$$?

Accepted Answer

$$[Cl^-] = \frac{1.8 	imes 10$$^{-10}$$}{1.0 	imes 10$$^{-2}$$} = 1.8 	imes 10$$^{-8}$$ M

Question 13

Which of the following best describes the process of selective precipitation in qualitative analysis?

Accepted Answer

Adding a counterion to selectively precipitate one ion while leaving others in solution based on differences in $$K_{sp}.$$

Question 14

Which of the following is the correct expression for the solubility product constant ($$K_{sp}) $$for $$CaF_2$$?

Accepted Answer

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

Question 15

A 0.10 M solution of $$NH_3$$ ($$K_b = 1.8 	imes 10$$^{-5}$$) is used to make a buffer with NH_4Cl$. What mass of NH_4Cl$ is needed to make 2.0 L of buffer at pH 9.00? (pK_b = 4.74$, pK_w = 14.00$)

Accepted Answer

First, calculate $$pK_a = 14.00 - 4.74 = 9.26. $$Use $$pH = pK_a + \log \left( \frac{[A^-]}{[HA]} \right) to $$solve for $$[NH_4$^+]$$, then multiply by 2.0 L and molar mass.

Question 16

Which of the following is true about the titration curve of a polyprotic acid such as $$H_3PO_3$$?

Accepted Answer

There are multiple equivalence points, each corresponding to the removal of a proton.

Question 17

Which of the following best explains why phenolphthalein is a suitable indicator for titrating a weak acid with a strong base?

Accepted Answer

The color change interval of phenolphthalein overlaps with the rapid pH change near the equivalence point.

Chapter 17: Additional Aqueous Equilibria – Study Notes

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