Q: A solution is prepared by dissolving 0.10 mol of citric acid ($H_3C_6H_5O_7$) in enough water to make 1.0 L of solution. Using $K_{a1} = 7.4 \times 10^{-4}$, which of the following is the best estimate for the initial $[H_3O^+]$ concentration?

$[H_3O^+] \approx \sqrt{K_{a1} \times 0.10}$

Question 1

Which of the following best describes the behavior of a weak acid in aqueous solution?

Accepted Answer

It partially dissociates, establishing a dynamic equilibrium between the acid and its ions.

Question 2

Given the equilibrium $\mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)}$, which expression correctly represents the acid dissociation constant $K_a$?

Accepted Answer

$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

Question 3

A 0.010 M solution of niacin ($K_a = 1.5 	imes 10^{-5}$) is prepared. What is the pH of this solution? (Assume $x \ll 0.010$)

Accepted Answer

pH = 3.08

Question 4

A student prepares a 0.10 M solution of formic acid and measures the pH to be 2.38 at 25°C. What is the value of $K_a$ for formic acid at this temperature?

Accepted Answer

$K_a = 4.2 	imes 10^{-4}$

Question 5

Which of the following acids is polyprotic and has the smallest $K_{a2}$ value according to Table 16.3?

Accepted Answer

Ascorbic acid

Question 6

What is the correct setup to calculate the pH of a 0.0037 M solution of carbonic acid ($H_2CO_3$) using $K_{a1} = 4.3 	imes 10^{-7}$?

Accepted Answer

Set up $K_a = \frac{[H_3O^+][HCO_3^-]}{[H_2CO_3]}$ and solve for $[H_3O^+]$

Question 7

Which of the following statements about the pH of weak acid solutions is correct?

Accepted Answer

The pH depends on both the concentration of the acid and its $K_a$ value.

Question 8

Which acid from Table 16.3 would you expect to have the highest initial concentration of $H_3O^+$ in a 0.10 M solution?

Accepted Answer

Sulfuric acid ($H_2SO_4$)

Question 9

If the $K_a$ of a weak acid increases, what happens to the pH of a solution of that acid at the same concentration?

Accepted Answer

The pH decreases.

Question 10

Which of the following best explains why polyprotic acids have multiple $K_a$ values?

Accepted Answer

Each proton is lost in a separate step, and each step has a different equilibrium constant.

Question 11

A solution is prepared by dissolving 0.10 mol of citric acid ($H_3C_6H_5O_7$) in enough water to make 1.0 L of solution. Using $K_{a1} = 7.4 	imes 10^{-4}$, which of the following is the best estimate for the initial $[H_3O^+]$ concentration?

Accepted Answer

$[H_3O^+] \approx \sqrt{K_{a1} 	imes 0.10}$

Question 12

Which of the following is a correct interpretation of the pH scale in the context of weak acid solutions?

Accepted Answer

pH is a logarithmic measure of the $[H_3O^+]$ concentration.

Question 13

A 0.10 M solution of oxalic acid ($K_{a1} = 5.9 	imes 10^{-2}$) is prepared. Which of the following best describes the expected pH compared to a 0.10 M solution of ascorbic acid ($K_{a1} = 8.0 	imes 10^{-5}$)?

Accepted Answer

The oxalic acid solution will have a lower pH.

Question 14

Which of the following is the correct formula to calculate the pH from the hydronium ion concentration?

Accepted Answer

$\mathrm{pH} = -\log_{10}[H_3O^+]$

Question 15

Which of the following best describes the relationship between $K_a$ and acid strength?

Accepted Answer

A larger $K_a$ value indicates a stronger acid.

Calculations Involving Weak Acids and Polyprotic Acids

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