Question 1

Given the equilibrium expression for the ionization of a weak acid $HA$:

$HA (aq) + H_2O (l) \rightleftharpoons A^- (aq) + H_3O^+ (aq)$

Which of the following correctly represents the acid dissociation constant $K_a$ for this reaction?

Accepted Answer

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

Question 2

If the $K_b$ of $NH_3$ is $1.76 	imes 10^{-5}$, what is the $K_a$ of its conjugate acid $NH_4^+$? (Given $K_w = 1.0 	imes 10^{-14}$ at 25°C)

Accepted Answer

$K_a = 5.68 	imes 10^{-10}$

Question 3

Which of the following statements is true regarding the relationship between $K_a$ and $pK_a$?

Accepted Answer

A higher $K_a$ value corresponds to a lower $pK_a$ value.

Question 4

Which Bronsted-Lowry base has the greatest concentration of hydroxide ions in solution?

a) $C_5H_5N$ ($K_b = 8.3 	imes 10^{-9}$)
b) $C_6H_7N$ ($K_b = 1.7 	imes 10^{-4}$)
c) $C_7H_9N$ ($K_b = 1.0 	imes 10^{-9}$)
d) $C_8H_{11}N$ ($K_b = 3.5 	imes 10^{-6}$)

Accepted Answer

$C_6H_7N$

Question 5

Which Bronsted-Lowry acid has the weakest conjugate base?

a) $HCNO$ ($K_a = 2.0 	imes 10^{-4}$)
b) $HF$ ($K_a = 3.5 	imes 10^{-4}$)
c) $NH_3$ ($K_a = 2.5 	imes 10^{-9}$)
d) $HCO_2$ ($K_a = 4.3 	imes 10^{-4}$)

Accepted Answer

$HF$

Question 6

If $HF$ has a higher $K_a$ than $CH_3COOH$, what can be said about the equilibrium position for the reaction:

$HF (aq) + CH_3COO^- (aq) \rightleftharpoons F^- (aq) + CH_3COOH (aq)$

Accepted Answer

Equilibrium lies to the right

Question 7

Which of the following acids will have the greatest concentration of $H_3O^+$ ions in solution at equal molarity?

a) $HA$ ($K_a = 1.0 	imes 10^{-7}$)
b) $HB$ ($K_a = 2.5 	imes 10^{-5}$)
c) $HC$ ($K_a = 3.0 	imes 10^{-9}$)
d) $HD$ ($K_a = 5.0 	imes 10^{-6}$)

Accepted Answer

$HB$

Question 8

A student mixes $NH_3$ and $NH_4^+$ in water. Which statement best describes the resulting solution?

Accepted Answer

It acts as a buffer, resisting changes in pH.

Question 9

Which of the following is true about the strength of a conjugate base?

Accepted Answer

The weaker the acid, the stronger its conjugate base.

Question 10

Calculate the $pK_a$ for an acid with $K_a = 2.5 	imes 10^{-6}$.

Accepted Answer

$pK_a = 5.60$

Question 11

Which of the following statements best explains why $HF$ is considered a weak acid despite having a relatively high $K_a$ compared to other weak acids?

Accepted Answer

It does not completely dissociate in water.

Question 12

If the $K_a$ of $HA$ is $1.0 	imes 10^{-8}$, what is the $K_b$ of its conjugate base $A^-$? ($K_w = 1.0 	imes 10^{-14}$)

Accepted Answer

$K_b = 1.0 	imes 10^{-6}$

Question 13

Which of the following pairs will form a buffer solution when mixed in water?

Accepted Answer

$CH_3COOH$ and $CH_3COO^-$

Question 14

A solution contains $C_6H_7N$ ($K_b = 1.7 	imes 10^{-4}$) and its conjugate acid. What is the $pH$ behavior of this solution upon addition of small amounts of strong acid?

Accepted Answer

The $pH$ will decrease only slightly.

Question 15

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

Accepted Answer

$K_a 	imes K_b = K_w$

Question 16

Which of the following acids will have the strongest conjugate base?

a) $HNO_3$ ($K_a = 1.0 	imes 10^1$)
b) $HCOOH$ ($K_a = 1.8 	imes 10^{-4}$)
c) $HCN$ ($K_a = 4.9 	imes 10^{-10}$)
d) $HF$ ($K_a = 3.5 	imes 10^{-4}$)

Accepted Answer

$HCN$

Question 17

A buffer solution is prepared using $CH_3COOH$ and $CH_3COO^-$. If a small amount of $HCl$ is added, what will happen to the $pH$?

Accepted Answer

The $pH$ will decrease slightly.

Question 18

Which of the following is NOT a property of a buffer solution?

Accepted Answer

It is made from a strong acid and a strong base.

Acid-Base Equilibria and Strengths in GOB Chemistry

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