Q: If the $$pK_a of an $$acid is 2.70, what is its $$K_a$$ value?

$$K_a = 5.01 \times 10$$^{-3}$$

Q: A solution of formic acid ($$HCOOH) $$has a $$K_a of$$ $$2.1 \times 10$$^{-4}$$. If the pH of the solution is 2.47, what is the concentration of $$[H_3O^+]$$?

$$[H_3O^+] = 3.39 \times 10$$^{-4}$$ M

Question 1

Which of the following statements best describes a weak acid?

Accepted Answer

A weak acid reacts only partially with water, resulting in an equilibrium between the acid and its ions.

Question 2

Which of the following is the correct equilibrium expression for a weak acid $$HA in $$water?

Accepted Answer

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

Question 3

If the $$pK_a of an $$acid is 2.70, what is its $$K_a$$ value?

Accepted Answer

$$K_a = 5.01 	imes 10$$^{-3}$$

Question 4

Which of the following is true regarding the difference between strong acids and weak acids?

Accepted Answer

Strong acids have a higher equilibrium concentration of $$H_3O^+$$ compared to weak acids at the same initial concentration.

Question 5

Which of the following is an example of an inorganic weak acid?

Accepted Answer

$$HNO_2$$

Question 6

Given a 0.10 M solution of acetic acid ($$K_a = 1.8 	imes 10$$^{-5}$$), which of the following is the correct setup for the K_a$ expression using an ICE table?

Accepted Answer

$$K_a = \frac{[x][x]}{0.10 - x}$$

Question 7

A solution of formic acid ($$HCOOH) $$has a $$K_a of$$ $$2.1 	imes 10$$^{-4}$$. If the pH of the solution is 2.47, what is the concentration of $$[H_3O^+]$$?

Accepted Answer

$$[H_3O^+] = 3.39 	imes 10$$^{-4}$$ M

Question 8

Which of the following statements about $$K_a$$ values is correct?

Accepted Answer

A larger $$K_a$$ value means the acid is stronger and dissociates more in water.

Question 9

Which of the following is the correct relationship between $$pK_a$$ and $$K_a$$?

Accepted Answer

$$pK_a = -\log K_a$$

Question 10

A student is asked to calculate the $$K_a of $$lactic acid given a 0.100 M solution with a pH of 2.08. What is the first step in solving this problem?

Accepted Answer

Use the pH to find $$[H_3O^+].$$

Question 11

Which of the following is a conjugate acid of a weak base?

Accepted Answer

$$NH_4$$^+$$

Question 12

If $$K_a$$ for $$HNO_2 is$$ $$5.56 	imes 10$$^{-4}$$, which of the following statements is true?

Accepted Answer

$$HNO_2 is a $$weak acid.

Question 13

Which of the following best explains why $$Al^{3+}$$ can act as a weak acid in water?

Accepted Answer

Because it is a metal ion with a high charge, it polarizes water molecules, releasing $$H^+.$$

Question 14

A 0.10 M solution of acetic acid ($$K_a = 1.8 	imes 10$$^{-5}$$) is prepared. Which assumption can simplify the calculation of $$[H_3O^+]$$?

Accepted Answer

Assume $$x is $$small compared to the initial concentration, so $$0.10 - x \approx 0.10.$$

Question 15

Which of the following is a correct step in solving for the pH of a weak acid solution using an ICE table?

Accepted Answer

Set the initial concentration of $$H_3O^+ to $$zero.

Question 16

If the $$pK_a of an $$acid is smaller, what does this indicate about the acid?

Accepted Answer

The acid is stronger.

Question 17

Which of the following is a real-world application of weak acids?

Accepted Answer

Acetic acid in vinegar used for food preservation.

Question 18

A solution contains 0.10 M $$HCN$$ ($$K_a = 6.2 	imes 10$$^{-10}$$). What is the pH$ of the solution? (Assume x$ is small compared to 0.10 M.)

Accepted Answer

$$pH = 9.2$$

Question 19

Which of the following is NOT a class of weak acids discussed in the materials?

Accepted Answer

Strong mineral acids

Weak Acids and Their Equilibria: Study Notes for General Chemistry

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