Q: A buffer is made by mixing 0.20 mol of benzoic acid (C\_6H\_5COOH) and 0.20 mol of sodium benzoate (NaC\_6H\_5COO) in 1.00 L of solution. The $K_a$ of benzoic acid is $6.31 \times 10^{-5}$. What is the pH of the buffer?

$\mathrm{pH} = -\log(6.31 \times 10^{-5}) = 4.20$

Q: A student titrates 30.0 mL of 0.50 M HNO\_3 with 1.00 M NaOH. What volume of NaOH is needed to reach the equivalence point?

$V_{\mathrm{NaOH}} = \frac{0.0300\ \mathrm{L} \times 0.50\ \mathrm{mol/L}}{1.00\ \mathrm{mol/L}} = 0.0150\ \mathrm{L} = 15.0\ \mathrm{mL}$

Question 1

Which of the following best describes the common-ion effect in a solution containing acetic acid (CH\_3COOH) and sodium acetate (CH\_3COONa)?

Accepted Answer

The presence of the common ion (CH\_3COO\^-) from sodium acetate suppresses the ionization of acetic acid, shifting the equilibrium to the left.

Question 2

A buffer solution is prepared by mixing 0.50 M acetic acid (CH\_3COOH) and 0.75 M sodium acetate (CH\_3COONa). The $K_a$ of acetic acid is $1.58 	imes 10^{-5}$. What is the pH of the buffer using the Henderson-Hasselbalch equation?

Accepted Answer

$\mathrm{pH} = 4.80 + \log \left( \frac{0.75}{0.50} \right) = 4.98$

Question 3

Which of the following combinations will NOT produce a buffer solution?

Accepted Answer

0.1 M HCl and 0.1 M NaCl

Question 4

A buffer is made by mixing 0.20 mol of benzoic acid (C\_6H\_5COOH) and 0.20 mol of sodium benzoate (NaC\_6H\_5COO) in 1.00 L of solution. The $K_a$ of benzoic acid is $6.31 	imes 10^{-5}$. What is the pH of the buffer?

Accepted Answer

$\mathrm{pH} = -\log(6.31 	imes 10^{-5}) = 4.20$

Question 5

Which statement about buffer capacity is correct?

Accepted Answer

Buffer capacity increases as the concentrations of the buffer components increase.

Question 6

A buffer solution contains 0.150 M propionic acid (C\_2H\_5COOH) and 0.100 M sodium propionate (NaC\_2H\_5COO) in 1.00 L. If 10.0 mL of 1.0 M NaOH is added, what will be the new concentration of propionic acid?

Accepted Answer

$0.150\ \mathrm{mol} - 0.010\ \mathrm{mol} = 0.140\ \mathrm{mol}$ in $1.01\ \mathrm{L}$, so $[\mathrm{C}_2\mathrm{H}_5\mathrm{COOH}] = \frac{0.140}{1.01} = 0.139\ \mathrm{M}$

Question 7

Which indicator would be most suitable for a titration with an equivalence point at pH 8.5?

Accepted Answer

Phenolphthalein (pH range 8.3–10.0)

Question 8

A student titrates 30.0 mL of 0.50 M HNO\_3 with 1.00 M NaOH. What volume of NaOH is needed to reach the equivalence point?

Accepted Answer

$V_{\mathrm{NaOH}} = \frac{0.0300\ \mathrm{L} 	imes 0.50\ \mathrm{mol/L}}{1.00\ \mathrm{mol/L}} = 0.0150\ \mathrm{L} = 15.0\ \mathrm{mL}$

Question 9

During the titration of a weak acid (HA) with a strong base, what is true about the pH at the equivalence point?

Accepted Answer

The pH is greater than 7 because the conjugate base of the weak acid hydrolyzes to produce OH\^-.

Question 10

A buffer solution is made by mixing 0.10 M acetic acid and 0.20 M sodium acetate in 1.00 L. $K_a$ for acetic acid is $1.58 	imes 10^{-5}$. What is the initial pH of the buffer?

Accepted Answer

$\mathrm{pH} = 4.80 + \log \left( \frac{0.20}{0.10} \right) = 5.10$

Question 11

Which of the following best explains why a buffer resists changes in pH upon addition of small amounts of strong acid or base?

Accepted Answer

The buffer contains a weak acid and its conjugate base, which react with added H\_3O\^+ or OH\^- to minimize pH changes.

Question 12

A student titrates 10.0 mL of 0.50 M hypochlorous acid (HCIO) with 0.20 M KOH. How many mL of KOH are needed to reach the equivalence point?

Accepted Answer

$V_{\mathrm{KOH}} = \frac{0.0100\ \mathrm{L} 	imes 0.50\ \mathrm{mol/L}}{0.20\ \mathrm{mol/L}} = 0.0250\ \mathrm{L} = 25.0\ \mathrm{mL}$

Question 13

After titrating 10.0 mL of 0.50 M HCIO with 25.0 mL of 0.20 M KOH, what is the pH at the equivalence point? ($K_a$ for HCIO is $3.0 	imes 10^{-8}$)

Accepted Answer

$\mathrm{pH} = 14 - (-\log(2.18 	imes 10^{-5})) = 10.34$

Question 14

Which of the following is true about the pH range in which a buffer is most effective?

Accepted Answer

A buffer is most effective when the pH is within $\pm 1$ of the buffer's $pK_a$.

Question 15

A buffer solution is prepared with 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

$\mathrm{pH} = 3.85 + \log \left( \frac{0.10}{0.12} \right) = 3.72$

Question 16

Which of the following best describes the effect of adding NH\_4Cl to a solution of NH\_3?

Accepted Answer

The concentration of NH\_4\^+ increases, and the equilibrium shifts to the left, decreasing OH\^- concentration.

Chapter 17: Additional Aspects of Aqueous Equilibria – Buffers, Common Ion Effect, and Titrations

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