Q: A solution of acetic acid ($CH_3COOH$) and sodium acetate ($CH_3COO^-$) is prepared. Which equation would you use to calculate the pH of this buffer solution?

$pH = pK_a + \log \frac{[\text{SALT}]}{[\text{ACID}]}$

Q: If the pH of a solution is 1.7, what is the concentration of hydronium ions $[H_3O^+]$ in the solution?

$[H_3O^+] = 0.0199\ \text{mol dm}^{-3}$

Question 1

Which of the following best describes a Brønsted-Lowry acid?

Accepted Answer

A substance that donates a proton ($H^+$) to another species.

Question 2

When hydrochloric acid ($HCl$) dissolves in water, what is the primary species formed as a result of the hydrogen bond between $HCl$ and $H_2O$?

Accepted Answer

$Cl^-$ and $H_3O^+$

Question 3

A solution of acetic acid ($CH_3COOH$) and sodium acetate ($CH_3COO^-$) is prepared. Which equation would you use to calculate the pH of this buffer solution?

Accepted Answer

$pH = pK_a + \log \frac{[	ext{SALT}]}{[	ext{ACID}]}$

Question 4

If the pH of a solution is 1.7, what is the concentration of hydronium ions $[H_3O^+]$ in the solution?

Accepted Answer

$[H_3O^+] = 0.0199\ 	ext{mol dm}^{-3}$

Question 5

Which of the following statements about weak acids is correct?

Accepted Answer

Weak acids only partially dissociate in water, so the solution contains a high proportion of the original acid molecules.

Question 6

A buffer solution is made by mixing $0.1$ mol of $CH_3COOH$ (acetic acid, $pK_a = 4.74$) and $0.1$ mol of $CH_3COO^-$ in $1$ L of water. What is the pH of the solution?

Accepted Answer

$pH = 4.74$

Question 7

Which of the following best explains why the rate of hydrogen gas evolution is greater when magnesium is added to a strong acid (like $HCl$) compared to a weak acid (like $CH_3COOH$) of the same concentration?

Accepted Answer

Strong acids have a higher concentration of hydronium ions ($H_3O^+$) than weak acids at the same concentration.

Question 8

What is the relationship between $pK_a$ and $pK_b$ for a conjugate acid-base pair at $25^	ext{o}C$?

Accepted Answer

$pK_a + pK_b = 14.00$

Question 9

Which of the following is a correct statement about buffer solutions?

Accepted Answer

A buffer is a mixture of weak conjugate acids and bases that stabilizes pH by accepting and providing protons.

Question 10

A solution contains $NH_3$ and $NH_4^+$. If the $pK_a$ of $NH_4^+$ is $9.25$, what is the pH of the solution when $[NH_3] = [NH_4^+]$?

Accepted Answer

$pH = 9.25$

Question 11

If the $pH$ of a solution is $5.0$, what is the $pOH$ of the solution at $25^	ext{o}C$?

Accepted Answer

$pOH = 9.0$

Question 12

Which of the following buffer systems would be most effective at maintaining a pH near 7.2?

Accepted Answer

$H_2PO_4^-/HPO_4^{2-}$ ($pK_a = 7.21$)

Question 13

A student adds $O^{2-}$ (oxide ion) to water. What is the resulting chemical equation for the reaction that occurs?

Accepted Answer

$O^{2-} + H_2O \rightarrow 2OH^-$

Question 14

Which of the following statements about logarithms and pH is correct?

Accepted Answer

The pH scale is logarithmic, so a change of 1 pH unit corresponds to a tenfold change in $[H_3O^+]$ concentration.

Question 15

A buffer solution is prepared with $[CH_3COOH] = 0.10$ M and $[CH_3COO^-] = 0.20$ M. If $pK_a = 4.74$, what is the pH of the solution?

Accepted Answer

$pH = 5.04$

Question 16

Which of the following best explains why buffer solutions resist changes in pH when small amounts of acid or base are added?

Accepted Answer

The weak acid neutralizes added base and the conjugate base neutralizes added acid.

Question 17

A solution is prepared by dissolving $0.1$ mol of $NH_3$ in water. Only about 1 in 100 $NH_3$ molecules are protonated. What does this indicate about ammonia as a base?

Accepted Answer

Ammonia is a weak base because it only partially accepts protons from water.

Question 18

If a buffer solution is made with $[HA] = 0.1$ M and $pK_a = 4.75$, using the approximation $pH = \frac{1}{2} pK_a - \frac{1}{2} \log C$, what is the pH?

Accepted Answer

$pH = 2.88$

Question 19

Which of the following is true regarding the effectiveness of a buffer?

Accepted Answer

A buffer is most effective when the concentrations of the weak acid and its conjugate base are approximately equal.

Acids, Bases, pH, and Biological Buffers: Key Concepts and Calculations

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