Q: Given a weak acid HA with $pK_a = 4.00$ and a formal concentration of 0.10 M, what is the fraction of dissociation ($\alpha$) of the acid if $x = 3.1 \times 10^{-3}$ M?

$\alpha = \dfrac{3.1 \times 10^{-3}}{0.10} = 0.031$

Q: A weak base B has a formal concentration of 0.0372 M and $K_b = 2.6 \times 10^{-5}$. What is the concentration of OH$^-$ at equilibrium?

$[OH^-] = x = 3.4 \times 10^{-3}$ M$

Question 1

Which of the following statements best describes a strong acid in aqueous solution?

Accepted Answer

It dissociates completely, so the concentration of hydronium ions equals the formal concentration of the acid.

Question 2

Calculate the pH of a 0.010 M HBr solution in 0.090 M KBr, given that the activity coefficient ($\gamma_{H^+}$) is 0.83.

Accepted Answer

$pH = -\log[(0.010)(0.83)] = 2.08$

Question 3

A student prepares a 1.0 x $10^{-8}$ M KOH solution. Why is the pH not simply calculated as $pH = 14 - (-8) = 22$?

Accepted Answer

Because the solution is so dilute that the contribution of water's autoionization to [OH$^-$] is significant.

Question 4

Which equation correctly relates the acid dissociation constant ($K_a$) and the base hydrolysis constant ($K_b$) for a conjugate acid-base pair in water?

Accepted Answer

$K_a 	imes K_b = K_w$

Question 5

A buffer is prepared by mixing 0.050 mol of HA and 0.050 mol of A$^-$. If 0.020 mol of strong acid is added, what will be the new ratio of [A$^-$]/[HA]?

Accepted Answer

0.030 / 0.070

Question 6

Which of the following best explains why the pH of a buffer solution is nearly independent of dilution?

Accepted Answer

Because the ratio of the concentrations of acid and base remains constant upon dilution.

Question 7

Given a weak acid HA with $pK_a = 4.00$ and a formal concentration of 0.10 M, what is the fraction of dissociation ($\alpha$) of the acid if $x = 3.1 	imes 10^{-3}$ M?

Accepted Answer

$\alpha = \dfrac{3.1 	imes 10^{-3}}{0.10} = 0.031$

Question 8

Which of the following is true regarding the relationship between acid strength and its conjugate base?

Accepted Answer

The stronger the acid, the weaker its conjugate base.

Question 9

A buffer solution is made by mixing 50 mL of 0.5 M sodium acetate and 25 mL of 0.1 M acetic acid. What is the pH of the buffer if $pK_a = 4.757$?

Accepted Answer

$pH = 4.757 + \log\left(\dfrac{0.333}{0.033}\right) = 5.757$

Question 10

Which statement about buffer capacity is correct?

Accepted Answer

Buffer capacity increases as the concentration of buffer components increases.

Question 11

A weak base B has a formal concentration of 0.0372 M and $K_b = 2.6 	imes 10^{-5}$. What is the concentration of OH$^-$ at equilibrium?

Accepted Answer

$[OH^-] = x = 3.4 	imes 10^{-3}$ M$

Question 12

Which of the following is the correct form of the Henderson-Hasselbalch equation for a buffer made from a weak acid HA and its conjugate base A$^-$?

Accepted Answer

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

Question 13

If the ratio $[A^-]/[HA]$ in a buffer changes by a factor of 10, how much does the pH change?

Accepted Answer

The pH changes by 1 unit.

Question 14

Which weak acid would be most appropriate for preparing a buffer with pH 10.0?

Accepted Answer

An acid with $pK_a$ close to 10.0, such as ammonia ($pK_a = 9.24$).

Question 15

A buffer solution contains 0.364 mol HA and 0.636 mol A$^-$. If 0.05 mol of strong acid is added, what is the new pH if $pK_a = 4.757$?

Accepted Answer

$pH = 4.757 + \log\left(\dfrac{0.636 - 0.05}{0.364 + 0.05}\right) = 4.908$

Question 16

Which of the following is true about the useful pH range of a buffer?

Accepted Answer

It is typically $pK_a \pm 1$.

Question 17

A solution contains 0.10 M HA ($pK_a = 4.00$) and 0.10 M A$^-$. What is the pH of the solution?

Accepted Answer

$pH = pK_a = 4.00$

Question 18

If a buffer solution contains 50 mmol HA and 50 mmol A$^-$, and 15 mmol of strong base is added, by how much will the pH rise?

Accepted Answer

By 0.27 pH units.

Monoprotic Acid-Base Equilibria: Strong and Weak Acids/Bases, Equilibrium Calculations, and Buffers

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