Question 1

Which of the following best describes a diprotic acid?

Accepted Answer

An acid that can donate two acidic hydrogens and thus has two $$K_a$$ values.

Question 2

Given the following dissociation steps for a generic diprotic acid $$H_2A$$:

$$H_2A \rightleftharpoons H^+ + HA^-$$  ($$K_{a1}$$)

$$HA^- \rightleftharpoons H^+ + A$$^{2-}$$  (K$$_{a2}$$)

Which species is the conjugate base after the first dissociation?

Accepted Answer

$$HA^-$$

Question 3

If the $$K_{a1}$$ for carbonic acid ($$H_2CO_3) is$$ $$4.3 	imes 10$$^{-7}$$ and K$$_{a2}$$ is 5.6$$ 	imes $$10^{-11}$$, what is the base dissociation constant $$K_b$$ for the reaction:

$$CO_3$$^{2-} + H_2O ightleftharpoons $$HCO_3^- + OH$$^-$$?

Accepted Answer

$$1.8 	imes 10$$^{-4}$$

Question 4

Which of the following correctly expresses the equilibrium constant $$K_a$$ for hydrofluoric acid ($$HF$$)?

Accepted Answer

$$K_a = \dfrac{[H^+][F^-]}{[HF]}$$

Question 5

Why is $$K_{a1}$$ generally much larger than $$K_{a2}$$ for diprotic acids?

Accepted Answer

The first proton is removed from a neutral molecule, while the second is removed from a negatively charged ion.

Question 6

Which of the following is the correct relationship between $$K_a$$ and $$K_b$$ for a conjugate acid-base pair?

Accepted Answer

$$K_a 	imes K_b = K_w$$

Question 7

A student mixes $$H_2CO_3 in $$water. Which of the following best describes the sequence of species formed as the acid dissociates?

Accepted Answer

$$H_2CO_3 \rightarrow $$HCO_3$$^- \rightarrow CO_3$^{2-}$$

Question 8

If $$K_{a1}$$ for a diprotic acid is $$1.0 	imes 10$$^{-3}$$ and K$$_{a2}$$ is 1.0$$ 	imes $$10^{-8}$$, which statement is true?

Accepted Answer

The first proton is much more easily lost than the second.

Question 9

Which of the following is the correct $$K_b$$ expression for the reaction $$CO_3$$^{2-} + H_2O \rightleftharpoons $$HCO_3^- + OH$$^-$$?

Accepted Answer

$$K_b = \dfrac{[$$HCO_3$$^-][OH^-]}{[CO_3$^{2-}]}$$

Question 10

A solution contains $$H_2CO_3$$, $$HCO_3$$^-$$, and CO_3$^{2-}. $$Which of the following would increase the concentration of $$CO_3$$^{2-}$$?

Accepted Answer

Adding a strong base to the solution.

Question 11

Which of the following best explains why $$K_{a2} is $$much smaller than $$K_{a1}$$ for $$H_2CO_3$$?

Accepted Answer

The negative charge on $$HCO_3$$^-$$ makes it harder to lose a second proton.

Question 12

If $$K_w = 1.0 	imes 10$$^{-14}$$ at 25°C, and K$$_{a2}$$ for H_2CO_3$ is 5.6$$ 	imes $$10^{-11}$$, what is the $$K_b$$ for $$CO_3$$^{2-}$$?

Accepted Answer

$$1.8 	imes 10$$^{-4}$$

Question 13

Which of the following is a correct statement about the relationship between $$K_{a1}$$ and $$K_{a2}$$ for a diprotic acid?

Accepted Answer

$$K_{a1} is $$always greater than $$K_{a2}.$$

Question 14

A chemist wants to prepare a buffer solution using $$H_2CO_3$$ and $$HCO_3$$^-$$. Which equilibrium constant should be considered for calculating the pH of the buffer?

Accepted Answer

$$K_{a1}$$

Question 15

Which of the following is the correct conjugate acid-base pair for the first dissociation of $$H_2CO_3$$?

Accepted Answer

$$H_2CO_3$$ and $$HCO_3$$^-$$

Diprotic Acids and Bases: Acid Dissociation and Equilibria

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