Q: If the $$K_a of HF is$$ $$3.5 \times 10$$^{-5}$$, what is the K_b$ of F$$^-$$?

$$K_b = \frac{K_w}{K_a} = \frac{1.0 \times 10$$^{-14}$$}{3.5 \times 10$$^{-5}$$}$$

Question 1

A 0.22 M NaF solution is prepared at 25°C. Given that the $$K_a of HF is$$ $$3.5 	imes 10$$^{-5}$$, what is the pH of the solution? (NaF is a salt of a weak acid and strong base.)

Accepted Answer

10.20

Question 2

Calculate the pH of a buffer solution containing $$0.225 M$$ $$HC_2H_3O_2$$ and $$0.162 M$$ $$NaC_2H_3O_2. $$The $$K_a$$ for $$HC_2H_3O_2 is$$ $$1.8 	imes 10$$^{-5}$$.

Accepted Answer

4.89

Question 3

For a buffer composed of an acid $$HA$$ and its conjugate base $$A^-$$, which combination would make the most effective buffer?

Accepted Answer

0.500\,M\,HA;\,0.500\,M\,A^-

Question 4

Which equation is used to calculate the pH of a buffer solution containing a weak acid and its conjugate base?

Accepted Answer

$$pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$$

Question 5

If the concentration of the conjugate base in a buffer is doubled while the acid concentration remains constant, what happens to the pH?

Accepted Answer

The pH increases

Question 6

A buffer solution is made by mixing equal concentrations of $$HC_2H_3O_2$$ and $$NaC_2H_3O_2. $$What is the pH of the buffer relative to the $$pK_a of$$ $$HC_2H_3O_2$$?

Accepted Answer

The pH equals the $$pK_a$$

Question 7

Which of the following best describes why a buffer is most effective when the concentrations of acid and conjugate base are equal?

Accepted Answer

The buffer has maximum capacity to resist pH changes

Question 8

A student adds a small amount of strong acid to a buffer solution. What happens to the pH?

Accepted Answer

The pH decreases slightly

Question 9

Given $$K_a$$ for $$HC_2H_3O_2 is$$ $$1.8 	imes 10$$^{-5}$$, what is the pK_a$?

Accepted Answer

$$pK_a = 4.74$$

Question 10

Which salt, when dissolved in water, will produce a basic solution?

Accepted Answer

NaF

Question 11

A buffer is prepared using $$0.225 M$$ $$HC_2H_3O_2$$ and $$0.162 M$$ $$NaC_2H_3O_2. If$$ $$0.01$$ mol of $$HCl is $$added to $$1 L of $$this buffer, what will happen to the pH?

Accepted Answer

The pH will decrease slightly

Question 12

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

Accepted Answer

It can resist large changes in pH from strong acids or bases

Question 13

What is the relationship between $$K_a$$ and $$K_b$$ for a conjugate acid-base pair?

Accepted Answer

$$K_a 	imes K_b = K_w$$

Question 14

If the $$K_a of HF is$$ $$3.5 	imes 10$$^{-5}$$, what is the K_b$ of F$$^-$$?

Accepted Answer

$$K_b = \frac{K_w}{K_a} = \frac{1.0 	imes 10$$^{-14}$$}{3.5 	imes 10$$^{-5}$$}$$

Question 15

Which of the following would NOT be suitable for making a buffer solution?

Accepted Answer

Strong acid and strong base

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