The student adds 5.0 mL of 0.10 M HCl to the 50.0 mL buffer. What is the new pH (approximately)?

Answer: 4.71 Initial moles: HA = A- = 0.0125. Add 5.0e-4 mol HCl converts A- to HA: A- = 0.0120, HA = 0.0130. pH = 4.74 + log(0.0120/0.0130) = 4.74 - 0.035 = 4.71.

Why does the dashed buffer (lower capacity) change pH faster?

Answer: It contains less HA and A- to absorb added base Buffer capacity scales with the absolute amount of weak acid and conjugate base. Lower amounts mean smaller capacity to absorb added strong acid/base, so pH shifts more.

A buffer is generally effective within

Answer: pKa ± 1 Within pKa ± 1, ratio of components stays between 1/10 and 10 — workable buffer range.

AP Chemistry · Topic 8.10

Buffer Capacity Practice

Part of Acids and Bases.(SAP-10.D)

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$A student prepares an acetic acid / acetate buffer at pH = 4.74 (pKa of CH3COOH = 4.74). Available stock solutions are 0.500 M CH3COOH and 0.500 M CH3COONa. The student combines 25.0 mL of acetic acid with 25.0 mL of sodium acetate to a total of 50.0 mL. The measured pH is 4.74.$
$The student adds 5.0 mL of 0.10 M HCl to the 50.0 mL buffer. What is the new pH (approximately)?$
- A
  $5.00$
- B
  $4.71$
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- C
  $4.04$
- D
  $1.30$
Why
$Initial moles: HA = A- = 0.0125. Add 5.0e-4 mol HCl converts A- to HA: A- = 0.0120, HA = 0.0130. pH = 4.74 + log(0.0120/0.0130) = 4.74 - 0.035 = 4.71.$
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Sample 2difficulty 3/5
$Why does the dashed buffer (lower capacity) change pH faster?$
- A
  $K is smaller for the dashed one$
- B
  $It contains less HA and A- to absorb added base$
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- C
  $It has a different pKa$
- D
  $It is at higher temperature$
Why
$Buffer capacity scales with the absolute amount of weak acid and conjugate base. Lower amounts mean smaller capacity to absorb added strong acid/base, so pH shifts more.$
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Sample 3difficulty 3/5
$A buffer is generally effective within$
- A
  $Any pH$
- B
  $Only at pKa exactly$
- C
  $pKa \pm 5$
- D
  $pKa \pm 1$
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Why
$Within pKa \pm 1, ratio of components stays between 1/10 and 10 — workable buffer range.$
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Sample 4difficulty 3/5
$Buffer capacity is greatest when$
- A
  $pH ≪ pKa$
- B
  $[A⁻] ≫ [HA]$
- C
  $Concentration is low$
- D
  $Components are at equal concentration AND total concentration is high$
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Why
$Both equal ratio (around pKa) and large absolute amounts give the most resilience.$
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Sample 5difficulty 4/5
$A buffer is most effective when$
- A
  $^-$
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- B
  $Only acid is present$
- C
  $pH = 7$
- D
  $^-$
Why
$_a$
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