Q: Which equation is most commonly used to calculate the pH of a buffer solution when the concentrations of the acid and its conjugate base are known?

$$\mathrm{pH = pK_a + \log\left(\frac{[\text{base}]}{[\text{acid}]}\right)}$$

Q: A buffer is prepared by adding ammonium chloride to a 0.500 M ammonia solution. If the desired pH is 9.20 and $$K_b$$ for ammonia is $$1.80 \times 10$$^{-5}$$, which formula would you use to determine the required amount of ammonium chloride?

$$\mathrm{pOH = pK_b + \log\left(\frac{[\text{salt}]}{[\text{base}]}\right)}$$

Question 1

Which of the following best describes the mechanism by which a buffer solution resists changes in pH when a small amount of strong acid is added?

Accepted Answer

The strong acid reacts with the conjugate base present in the buffer, forming more weak acid.

Question 2

A buffer solution is prepared by mixing 0.30 mol of acetic acid ($$\mathrm{CH_3COOH}$$, $$K_a = 1.8 	imes 10$$^{-5}$$) and 0.30 mol of sodium acetate in enough water to make 1.0 L of solution. What is the pH of this buffer?

Accepted Answer

pH = $$4.74$$

Question 3

Which equation is most commonly used to calculate the pH of a buffer solution when the concentrations of the acid and its conjugate base are known?

Accepted Answer

$$\mathrm{pH = pK_a + \log\left(\frac{[	ext{base}]}{[	ext{acid}]}ight)}$$

Question 4

A buffer is made by mixing 0.12 M lactic acid ($$K_a = 1.4 	imes 10$$^{-4}$$) and 0.10 M sodium lactate. What is the pH of the buffer?

Accepted Answer

pH = $$3.90$$

Question 5

Which of the following statements about buffer capacity is correct?

Accepted Answer

Buffer capacity is highest when the concentrations of acid and conjugate base are both high and nearly equal.

Question 6

Suppose you dilute a buffer composed of 0.25 M acetic acid and 0.25 M acetate by a factor of 2. What happens to the pH?

Accepted Answer

The pH remains nearly unchanged.

Question 7

A buffer is prepared by adding ammonium chloride to a 0.500 M ammonia solution. If the desired pH is 9.20 and $$K_b$$ for ammonia is $$1.80 	imes 10$$^{-5}$$, which formula would you use to determine the required amount of ammonium chloride?

Accepted Answer

$$\mathrm{pOH = pK_b + \log\left(\frac{[	ext{salt}]}{[	ext{base}]}ight)}$$

Question 8

Which of the following is NOT a method for preparing a buffer solution?

Accepted Answer

Mixing a strong acid with a strong base.

Question 9

You want to prepare an acetate buffer (pKa = 4.74) at pH 7.00 with an acid concentration of 0.010 M. What is the required concentration of acetate ion?

Accepted Answer

$$1.8\ \mathrm{M}$$

Question 10

Which of the following best explains why the effective pH range of a buffer is typically within one unit of its pKa?

Accepted Answer

Outside this range, the ratio of acid to base becomes too extreme, reducing the buffer's ability to neutralize added acid or base.

Question 11

A buffer solution contains 0.20 M NH$$_3$$ and 0.15 M NH$$_4^+$$ ($$K_b = 1.8 	imes 10$$^{-5}$$). What is the pH of the solution?

Accepted Answer

pH = $$9.18$$

Question 12

Which of the following scenarios would most likely cause a buffer to lose its effectiveness?

Accepted Answer

Adding a large amount of strong base.

Question 13

If a buffer solution is composed of a weak acid and its conjugate base, which of the following will occur if a small amount of strong base is added?

Accepted Answer

The strong base reacts with the weak acid, forming more conjugate base.

Question 14

A buffer solution is made by mixing 0.10 mol of a weak acid (HA) and 0.15 mol of its conjugate base (A$$^-) in 1.0 L of $$water. If the $$K_a of HA is$$ $$2.0 	imes 10$$^{-5}$$, what is the pH of the solution?

Accepted Answer

pH = $$4.87$$

Question 15

Which of the following best describes the common ion effect in the context of buffer solutions?

Accepted Answer

The addition of a common ion suppresses the ionization of the weak acid, stabilizing the pH.

Question 16

A buffer solution is prepared by titrating a weak acid with a strong base. At what point during the titration is the buffer most effective?

Accepted Answer

When the concentrations of the weak acid and its conjugate base are equal.

Question 17

Which of the following is a real-world application of buffer solutions?

Accepted Answer

Maintaining blood pH in humans.

Question 18

A buffer solution is made by mixing 0.20 M acetic acid and 0.20 M sodium acetate. If the $$K_a of $$acetic acid is $$1.8 	imes 10$$^{-5}$$, what is the pH of the solution?

Accepted Answer

pH = $$4.74$$

Buffer Solutions: Principles, Calculations, and Applications

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