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

$$\mathrm{pH} = \mathrm{p}K_a + \log \left( \dfrac{[\text{A}^-]}{[\text{HA}]} \right)$$

Question 1

A buffer is prepared by dissolving 11.52 g of sodium propionate ($$\mathrm{C_2H_5COONa}) in 1.00 L of $$solution containing 0.090 mol of propionic acid ($$\mathrm{C_2H_5COOH}). $$Given $$K_a = 1.3 	imes 10$$^{-5}$$ for propionic acid, what is the pH of the buffer? (Molar mass of sodium propionate = 96.06 g/mol)

Accepted Answer

$$\mathrm{pH} = 5.13$$

Question 2

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

Accepted Answer

$$\mathrm{pH} = \mathrm{p}K_a + \log \left( \dfrac{[	ext{A}^-]}{[	ext{HA}]} ight)$$

Question 3

After adding 0.020 mol KOH to the buffer described above (no volume change), what is the new pH? (Assume all KOH reacts with the acid component.)

Accepted Answer

$$\mathrm{pH} = 5.34$$

Question 4

What is the effect of adding a strong acid, such as HBr, to the buffer solution described above?

Accepted Answer

The pH decreases slightly due to the buffer's resistance to pH change.

Question 5

Calculate the pH of the original buffer after the addition of 0.010 mol HBr. (Assume no volume change and complete reaction with the base component.)

Accepted Answer

$$\mathrm{pH} = 4.98$$

Question 6

Which of the following best describes the role of sodium propionate in the buffer system?

Accepted Answer

It acts as the conjugate base, neutralizing added acids.

Question 7

If the buffer solution described above is diluted to 2.00 L, what happens to the pH?

Accepted Answer

The pH remains nearly unchanged because the ratio of acid to base stays the same.

Question 8

Which of the following statements about buffer capacity is correct?

Accepted Answer

Buffer capacity is highest when the concentrations of acid and base are equal.

Question 9

A buffer solution is made using a weak acid and its salt. Which of the following will NOT increase the buffer's capacity?

Accepted Answer

Diluting the buffer with water.

Question 10

Which of the following best explains why buffers resist changes in pH upon addition of small amounts of strong acid or base?

Accepted Answer

The weak acid and its conjugate base react with added acid or base to minimize pH change.

Question 11

If a buffer is made from 0.10 mol of a weak acid (HA) and 0.10 mol of its conjugate base (A-) in 1.00 L, and $$K_a = 1.0 	imes 10$$^{-5}$$, what is the pH?

Accepted Answer

$$\mathrm{pH} = 5.00$$

Question 12

Which of the following is the correct expression for the acid dissociation constant ($$K_a) $$for propionic acid ($$\mathrm{C_2H_5COOH}$$)?

Accepted Answer

$$K_a = \dfrac{[\mathrm{C_2H_5COO^-}][H^+]}{[\mathrm{C_2H_5COOH}]}$$

Question 13

A buffer solution is most effective at resisting pH changes when:

Accepted Answer

The pH is close to the $$pK_a of $$the weak acid.

Question 14

Which of the following would destroy the buffer capacity of the solution described above?

Accepted Answer

Adding a large amount of strong acid or base.

Question 15

If the $$K_a of $$propionic acid is $$1.3 	imes 10$$^{-5}$$, what is its pK_a$?

Accepted Answer

$$pK_a = 4.89$$

Question 16

Which of the following best describes the chemical reaction that occurs when KOH is added to the buffer solution?

Accepted Answer

$$\mathrm{C_2H_5COOH} + OH^- \rightarrow \mathrm{C_2H_5COO^-} + H_2O$$

Question 17

A buffer solution contains 0.090 mol of propionic acid and 0.120 mol of sodium propionate in 1.00 L. What is the pH? ($$K_a = 1.3 	imes 10$$^{-5}$$)

Accepted Answer

$$\mathrm{pH} = 5.23$$

Question 18

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

Accepted Answer

It can maintain its pH indefinitely regardless of how much acid or base is added.

Buffer Solutions and pH Calculations: Acid-Base Equilibria

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