Q: Which equation is most useful for calculating the pH of a buffer solution containing a weak acid and its conjugate base?

$$\mathrm{pH} = \mathrm{p}K_a + \log\left(\dfrac{[\text{Base}]}{[\text{Acid}]}\right)$$

Q: Which of the following best explains why a buffer with $$\mathrm{p}K_a$$ far from the desired pH is less effective?

The ratio $$[\text{Base}]/[\text{Acid}] is $$far from 1, so the buffer cannot resist pH changes as effectively.

Question 1

Which of the following best describes a buffer solution?

Accepted Answer

A solution containing significant amounts of both a weak acid and its conjugate base that resists changes in pH when small amounts of acid or base are added.

Question 2

A buffer is prepared by mixing 0.100 M acetic acid ($$\mathrm{CH_3COOH}) $$and 0.100 M sodium acetate ($$\mathrm{CH_3COONa}). $$Given $$K_a = 1.8 	imes 10$$^{-5}$$ for acetic acid, what is the pH of the buffer?

Accepted Answer

$$\mathrm{pH} = 4.74$$

Question 3

Which equation is most useful for calculating the pH of a buffer solution containing a weak acid and its conjugate base?

Accepted Answer

$$\mathrm{pH} = \mathrm{p}K_a + \log\left(\dfrac{[	ext{Base}]}{[	ext{Acid}]}ight)$$

Question 4

A buffer is made by mixing 0.50 M HA and 0.50 M A$$^-$$, where $$\mathrm{p}K_a = 3.00. $$What is the pH after adding 0.10 moles of HCl to 1.00 L of this buffer?

Accepted Answer

$$\mathrm{pH} = 2.82$$

Question 5

Why is it important to choose a weak acid with a $$\mathrm{p}K_a$$ close to the desired pH when preparing a buffer?

Accepted Answer

Because the buffer will have the highest capacity and most effective pH control when $$\mathrm{pH} \approx \mathrm{p}K_a.$$

Question 6

A buffer is prepared using a weak acid with $$\mathrm{p}K_a = 4.30 to $$achieve a pH of 4.00. What is the required ratio of $$[	ext{Base}]/[	ext{Acid}]$$?

Accepted Answer

$$0.50$$

Question 7

Which of the following best describes the effective buffer range for a weak acid buffer system?

Accepted Answer

$$\mathrm{p}K_a \pm 1$$

Question 8

A buffer is made by mixing 1.0 L of 0.20 M HBrO and 1.0 L of 0.56 M NaBrO. If the final concentrations are $$[	ext{Acid}] = 0.10 M $$and $$[	ext{Base}] = 0.28 M$$, what is the ratio $$[	ext{Base}]/[	ext{Acid}]$$?

Accepted Answer

$$2.8$$

Question 9

Suppose you have two buffer solutions, both with pH = 4.00, but one has $$[	ext{Acid}] = 0.20 M $$and $$[	ext{Base}] = 0.10 M$$, while the other has $$[	ext{Acid}] = 0.60 M $$and $$[	ext{Base}] = 0.30 M. If 0.02 $$moles of NaOH are added to each (in 1.0 L), which buffer will experience the smallest change in pH and why?

Accepted Answer

The buffer with higher concentrations, because higher buffer component concentrations increase buffer capacity.

Question 10

What is buffer capacity?

Accepted Answer

The amount of strong acid or base a buffer can neutralize before the pH changes significantly.

Question 11

A buffer is prepared with $$[	ext{Base}] = 0.18 M $$and $$[	ext{Acid}] = 0.10 M$$, and $$\mathrm{p}K_a = 3.7447. $$What is the maximum amount of strong acid (in moles) that can be added to 1 L of this buffer before it loses its buffering ability?

Accepted Answer

$$0.15$$ moles

Question 12

A buffer is made with $$[	ext{Base}] = 0.18 M $$and $$[	ext{Acid}] = 0.10 M. $$What is the maximum amount of strong base (in moles) that can be added to 1 L of this buffer before it loses its buffering ability?

Accepted Answer

$$0.075$$ moles

Question 13

Which of the following statements about buffer solutions is FALSE?

Accepted Answer

A buffer can be made from a strong acid and its conjugate base.

Question 14

If you want to prepare a buffer with pH = 9.00, which of the following would be the best choice for the weak acid or base component?

Accepted Answer

A weak acid or base whose $$\mathrm{p}K_a is $$close to 9.00

Question 15

A buffer is prepared with $$[	ext{Base}] = 0.13 M $$and $$[	ext{Acid}] = 0.27 M$$, and $$\mathrm{p}K_a = 4.30. $$What is the pH of this buffer?

Accepted Answer

$$4.00$$

Question 16

After adding 0.010 moles of NaOH to 1.0 L of a buffer with $$[	ext{Base}] = 0.13 M $$and $$[	ext{Acid}] = 0.27 M$$, what is the new pH? ($$\mathrm{p}K_a = 4.30)$$

Accepted Answer

$$4.03$$

Question 17

Which of the following best explains why a buffer with $$\mathrm{p}K_a$$ far from the desired pH is less effective?

Accepted Answer

The ratio $$[	ext{Base}]/[	ext{Acid}] is $$far from 1, so the buffer cannot resist pH changes as effectively.

Question 18

A buffer is prepared with $$[	ext{Base}] = 0.04 M $$and $$[	ext{Acid}] = 0.36 M$$, and $$\mathrm{p}K_a = 5.00. $$What is the pH of this buffer?

Accepted Answer

$$4.00$$

Question 19

Which of the following is NOT a step in preparing a buffer with a desired pH?

Accepted Answer

Choose a strong acid with a $$\mathrm{p}K_a$$ far from the desired pH.

Aqueous Ionic Equilibrium: Buffers and Buffer Capacity

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