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

$$K_a = 5.68 \times 10$$^{-10}$$

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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