CHEM134 C002 Spring 2026 | Digital
Using Buffers – Fully Solved 100% - APUS
SECTION 1: BUFFER DEFINITION & COMPOSITION (Questions 1–5)
Q1: Which of the following pairs of substances can form an effective buffer
solution?
A. HCl and NaCl
B. NaOH and NaCl
C. CH₃COOH and CH₃COONa
D. H₂SO₄ and NaHSO₄
Correct Answer: C
Rationale: Correct because a buffer requires a weak acid and its conjugate base
(or weak base and conjugate acid). Acetic acid (CH₃COOH) is a weak acid and
sodium acetate (CH₃COONa) provides the conjugate base CH₃COO⁻, forming an
effective buffer system.
Q2: When a small amount of strong acid is added to an acetate buffer
(CH₃COOH/CH₃COO⁻), what is the primary reaction that prevents a large pH
change?
A. CH₃COOH + H⁺ → CH₃COOH₂⁺
B. CH₃COO⁻ + H⁺ → CH₃COOH
C. CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
D. H⁺ + OH⁻ → H₂O
Correct Answer: B
Rationale: Correct because the conjugate base CH₃COO⁻ reacts with added H⁺ to
form the weak acid CH₃COOH, consuming the added strong acid and minimizing
pH change.
Q3: A buffer solution is composed of a weak base B and its conjugate acid BH⁺.
When a small amount of strong base (OH⁻) is added, which species reacts to
maintain pH stability?
A. B + OH⁻ → BOH
, B. BH⁺ + OH⁻ → B + H₂O
C. B + H⁺ → BH⁺
D. BH⁺ + H⁺ → BH₂²⁺
Correct Answer: B
Rationale: Correct because the conjugate acid BH⁺ reacts with added OH⁻ to form
the weak base B and water, consuming the added strong base and preventing a
significant pH increase.
Q4: For a buffer to effectively resist pH change upon addition of acid or base,
which condition must be met?
A. The buffer must contain only a weak acid
B. The concentrations of weak acid and conjugate base must be much larger than
the amount of added H⁺ or OH⁻
C. The buffer must have a pH of exactly 7.00
D. The buffer must contain a strong acid and strong base
Correct Answer: B
Rationale: Correct because buffer effectiveness requires that the reservoir
concentrations of both buffer components be significantly greater than the
amount of added strong acid or base, ensuring the ratio [base]/[acid] changes
minimally.
Q5: Which statement correctly describes how a buffer resists pH change upon
dilution with pure water?
A. The pH increases because the acid is diluted more than the base
B. The pH decreases because the base is diluted more than the acid
C. The pH remains essentially unchanged because both acid and base are diluted
equally, preserving the [base]/[acid] ratio
D. The pH changes unpredictably depending on the buffer system
Correct Answer: C
Rationale: Correct because dilution affects both the weak acid and conjugate
base equally, so the [base]/[acid] ratio remains constant and the pH does not
change, though buffer capacity decreases due to lower absolute concentrations.
Using Buffers – Fully Solved 100% - APUS
SECTION 1: BUFFER DEFINITION & COMPOSITION (Questions 1–5)
Q1: Which of the following pairs of substances can form an effective buffer
solution?
A. HCl and NaCl
B. NaOH and NaCl
C. CH₃COOH and CH₃COONa
D. H₂SO₄ and NaHSO₄
Correct Answer: C
Rationale: Correct because a buffer requires a weak acid and its conjugate base
(or weak base and conjugate acid). Acetic acid (CH₃COOH) is a weak acid and
sodium acetate (CH₃COONa) provides the conjugate base CH₃COO⁻, forming an
effective buffer system.
Q2: When a small amount of strong acid is added to an acetate buffer
(CH₃COOH/CH₃COO⁻), what is the primary reaction that prevents a large pH
change?
A. CH₃COOH + H⁺ → CH₃COOH₂⁺
B. CH₃COO⁻ + H⁺ → CH₃COOH
C. CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
D. H⁺ + OH⁻ → H₂O
Correct Answer: B
Rationale: Correct because the conjugate base CH₃COO⁻ reacts with added H⁺ to
form the weak acid CH₃COOH, consuming the added strong acid and minimizing
pH change.
Q3: A buffer solution is composed of a weak base B and its conjugate acid BH⁺.
When a small amount of strong base (OH⁻) is added, which species reacts to
maintain pH stability?
A. B + OH⁻ → BOH
, B. BH⁺ + OH⁻ → B + H₂O
C. B + H⁺ → BH⁺
D. BH⁺ + H⁺ → BH₂²⁺
Correct Answer: B
Rationale: Correct because the conjugate acid BH⁺ reacts with added OH⁻ to form
the weak base B and water, consuming the added strong base and preventing a
significant pH increase.
Q4: For a buffer to effectively resist pH change upon addition of acid or base,
which condition must be met?
A. The buffer must contain only a weak acid
B. The concentrations of weak acid and conjugate base must be much larger than
the amount of added H⁺ or OH⁻
C. The buffer must have a pH of exactly 7.00
D. The buffer must contain a strong acid and strong base
Correct Answer: B
Rationale: Correct because buffer effectiveness requires that the reservoir
concentrations of both buffer components be significantly greater than the
amount of added strong acid or base, ensuring the ratio [base]/[acid] changes
minimally.
Q5: Which statement correctly describes how a buffer resists pH change upon
dilution with pure water?
A. The pH increases because the acid is diluted more than the base
B. The pH decreases because the base is diluted more than the acid
C. The pH remains essentially unchanged because both acid and base are diluted
equally, preserving the [base]/[acid] ratio
D. The pH changes unpredictably depending on the buffer system
Correct Answer: C
Rationale: Correct because dilution affects both the weak acid and conjugate
base equally, so the [base]/[acid] ratio remains constant and the pH does not
change, though buffer capacity decreases due to lower absolute concentrations.
