Buffer Capacity

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AP Chemistry › Buffer Capacity

Questions 1 - 10
1

A student prepares two buffers using the same weak acid/conjugate base pair. Equal volumes of each buffer are placed in beakers.

  • Buffer P is made with moderate concentrations of both $\mathrm{HA}$ and $\mathrm{A^-}$.
  • Buffer Q is made with much higher concentrations of both $\mathrm{HA}$ and $\mathrm{A^-}$.

The student adds the same small amount of strong base to both beakers. Which claim best describes which buffer has greater capacity and the key reason?

Buffer P, because lower concentrations reduce the extent of reaction with added base

Buffer P, because its pH begins closer to the $pK_a$ than Buffer Q

Buffer Q, because it has more total moles of $\mathrm{HA}$ available to neutralize added $\mathrm{OH^-}$

Both buffers, because equal volumes always have equal buffer capacity

Both buffers, because buffer capacity depends only on the $\mathrm{A^-:HA}$ ratio, not the total amount

Explanation

Buffer capacity measures a solution's ability to minimize pH changes upon acid or base addition. It depends on the total moles of buffering agents available to react with and neutralize H⁺ or OH⁻, beyond just their concentration ratio. Higher total amounts allow the buffer to handle more addition before capacity is exceeded. While ratios affect pH positioning, capacity scales directly with the quantity of components. A tempting distractor is that equal volumes imply equal capacity, but this disregards differences in concentrations and total moles. To compare capacities, evaluate total buffering moles, as higher total concentration means greater ability to absorb added acid or base.

2

A student prepares two buffers in separate beakers, each containing a weak acid and its conjugate base:

  • Buffer M: moderate total amount of $\mathrm{HA}$ and $\mathrm{A^-}$ (moderately concentrated)
  • Buffer N: much larger total amount of $\mathrm{HA}$ and $\mathrm{A^-}$ (more concentrated)

Both buffers are prepared so that the ratio $\mathrm{A^-}/\mathrm{HA}$ is the same in each beaker. The student adds the same small amount of strong acid to both. Which conclusion is most appropriate?

Buffer M has greater capacity because its initial pH must be lower than Buffer N

Buffer M has greater capacity because lower concentration minimizes reaction with added acid

Both buffers have equal capacity because the $[\mathrm{A^-}]/[\mathrm{HA}]$ ratio is the same

Both buffers have equal capacity because the identity of the conjugate pair fixes the capacity

Buffer N has greater capacity because it contains more total buffering species to convert added $\mathrm{H^+}$ into $\mathrm{HA}$

Explanation

Buffer capacity quantifies resistance to pH changes in buffers upon acid or base addition. It depends on the total amount of buffering species available for neutralization, independent of their ratio alone. Buffers with identical ratios but different totals vary in capacity, with higher totals being superior. The ratio fixes the pH, but total concentration determines neutralization extent. A common misconception is that same ratios mean equal capacity, but this fails to account for differences in total moles. When evaluating buffers, compare total concentrations, as higher total amounts provide greater capacity to absorb added acid or base.

3

A student compares two buffers and then adds the same small amount of strong base to each.

Buffer A: $\text{HNO}_2/\text{NO}_2^-$ with $0.40,\text{M}$ $\text{HNO}_2$ and $0.40,\text{M}$ $\text{NaNO}_2$.

Buffer B: $\text{HNO}_2/\text{NO}_2^-$ with $0.10,\text{M}$ $\text{HNO}_2$ and $0.70,\text{M}$ $\text{NaNO}_2$.

Which buffer will show the smaller pH change?

Buffer A, because it contains more weak acid to neutralize added base

Buffer B, because buffers are most effective when one component is in large excess

They will change equally because the total concentration of buffer components is the same

Buffer B, because it contains more conjugate base

They will change equally because both use the same conjugate pair

Explanation

This question tests your understanding of buffer capacity. Buffer capacity indicates resistance to added base, relying on the total concentrations of weak acid and conjugate base to neutralize OH⁻. Higher weak acid concentrations better handle base additions. Capacity depends on these levels and the ratio's proximity to 1:1. A tempting distractor is that they will change equally because the total concentration of buffer components is the same, but Buffer A's balanced ratio and higher acid give better capacity. A transferable strategy is, for base additions, to seek higher weak acid concentration and balanced ratio; this means greater capacity to absorb added OH⁻.

4

A student prepares two buffers from different conjugate pairs.

Buffer A: $\text{HSO}_3^- / \text{SO}_3^{2-}$ with $0.15,\text{M}$ $\text{HSO}_3^-$ and $0.15,\text{M}$ $\text{SO}_3^{2-}$.

Buffer B: $\text{H}_2\text{PO}_4^- / \text{HPO}_4^{2-}$ with $0.30,\text{M}$ $\text{H}_2\text{PO}_4^-$ and $0.30,\text{M}$ $\text{HPO}_4^{2-}$.

The same small amount of strong acid is added to each buffer. Which buffer has greater capacity to resist the pH change?

Buffer A, because lower concentration buffers change pH less

They have equal capacity because both are prepared with equal conjugate pair concentrations

Buffer A, because sulfite-based buffers are inherently stronger than phosphate-based buffers

They have equal capacity because the same amount of strong acid is added

Buffer B, because it contains a greater amount of both buffer components

Explanation

This question tests your understanding of buffer capacity. Buffer capacity measures a buffer's ability to resist acid additions, relying on total concentrations of the buffer pair to neutralize H⁺. Higher concentrations mean more effective neutralization with less pH change. Capacity depends on these levels, independent of the specific pair for similar conditions. A tempting distractor is that they have equal capacity because both are prepared with equal conjugate pair concentrations, but this misses Buffer B's doubled concentrations providing greater capacity. A transferable strategy is to evaluate total buffer concentrations across different pairs; higher total concentration means greater capacity to absorb added acid or base.

5

A student prepares two buffers using the same weak acid/conjugate base pair and then adds the same small amount of strong acid to each.

Buffer A: prepared by mixing equal volumes of $0.050,\text{M}$ HA and $0.050,\text{M}$ NaA.

Buffer B: prepared by mixing equal volumes of $0.200,\text{M}$ HA and $0.200,\text{M}$ NaA.

Which statement best identifies which buffer has greater capacity to resist the pH change?

They have equal capacity because their HA:$A^{-}$ ratios are equal

Buffer A, because lower concentration means smaller pH change for any reaction

Buffer B, because it contains more moles of both HA and $A^{-}$ per liter

They have equal capacity because the same amount of strong acid is added

Buffer A, because dilute solutions have fewer particles that can change pH

Explanation

This question tests your understanding of buffer capacity. Buffer capacity measures resistance to pH change from added acid, based on total concentrations of weak acid and conjugate base to neutralize $H^{+}$. Higher concentrations allow greater neutralization without large shifts. Capacity hinges on these levels, not merely the ratio. A tempting distractor is that they have equal capacity because their HA:$A^{-}$ ratios are equal, but this is wrong as Buffer B's higher concentrations provide better capacity. A transferable strategy is to compare total concentrations of buffer components; higher total concentration means greater capacity to absorb added acid or base.

6

Two buffers are made in separate beakers.

Buffer 1: $\text{NH}_3/\text{NH}_4^+$ with $0.30,\text{M}$ $\text{NH}_3$ and $0.10,\text{M}$ $\text{NH}_4\text{Cl}$.

Buffer 2: $\text{NH}_3/\text{NH}_4^+$ with $0.10,\text{M}$ $\text{NH}_3$ and $0.30,\text{M}$ $\text{NH}_4\text{Cl}$.

The same small amount of strong base is added to each buffer. Which buffer will show the smaller pH change?

They will change equally because the total concentration is the same

Buffer 1, because buffers resist all additions of base equally well

Buffer 1, because it has more weak base

Buffer 2, because it has more conjugate acid available to neutralize added base

They will change equally because both contain the same conjugate pair

Explanation

This question tests your understanding of buffer capacity. Buffer capacity indicates how well a buffer handles added base, depending on the total concentrations of weak base and conjugate acid available to neutralize OH⁻. More conjugate acid better resists base by converting to base. It's the absolute amounts, not just ratio, that matter. A tempting distractor is that they will change equally because the total concentration is the same, but this ignores Buffer 2's higher conjugate acid better suiting base addition. A transferable strategy is, for base additions, to look for higher conjugate acid concentration; this means greater capacity to absorb added OH⁻.

7

Two buffers are prepared to the same final volume.

Buffer 1: $\text{H}_2\text{S}/\text{HS}^-$ with $0.15,\text{mol}$ $\text{H}_2\text{S}$ and $0.15,\text{mol}$ NaHS.

Buffer 2: $\text{H}_2\text{S}/\text{HS}^-$ with $0.30,\text{mol}$ $\text{H}_2\text{S}$ and $0.05,\text{mol}$ NaHS.

The same small amount of strong acid is added to each buffer. Which buffer will better resist the pH change?

Buffer 2, because unequal amounts of components always increase buffer capacity

They resist equally because both contain the same total moles of buffer components

They resist equally because both contain a weak acid and its conjugate base

Buffer 2, because it has more weak acid overall

Buffer 1, because it has more conjugate base available to consume added acid

Explanation

This question tests your understanding of buffer capacity. Buffer capacity assesses a buffer's ability to resist added acid, depending on the total moles of conjugate base available to neutralize H⁺. Greater moles mean more acid can be handled with minimal ratio change. It's these absolute amounts, not just the ratio, that define capacity. A tempting distractor is that they resist equally because both contain the same total moles of buffer components, but this overlooks Buffer 1's higher conjugate base moles offering superior resistance. A transferable strategy is, when adding acid, to evaluate the amount of conjugate base; higher amounts mean greater capacity to absorb added H⁺.

8

Two buffers are prepared, each with the same total concentration of buffer components.

Buffer 1: $\text{H}_2\text{CO}_3/\text{HCO}_3^-$ with $0.40,\text{M}$ $\text{H}_2\text{CO}_3$ and $0.10,\text{M}$ $\text{NaHCO}_3$.

Buffer 2: $\text{H}_2\text{CO}_3/\text{HCO}_3^-$ with $0.25,\text{M}$ $\text{H}_2\text{CO}_3$ and $0.25,\text{M}$ $\text{NaHCO}_3$.

A small, equal amount of strong acid is added to each buffer. Which buffer is expected to show the smaller pH change?

Buffer 1, because unequal concentrations always increase buffer capacity

Buffer 1, because it starts with more weak acid

Buffer 2, because it has more conjugate base available to consume added acid

They will change equally because their total buffer concentration is the same

They will change equally because they contain the same conjugate pair

Explanation

This question tests your understanding of buffer capacity. Buffer capacity describes a buffer's resistance to pH change from added acid, depending on the total amount of conjugate base available to neutralize H⁺, alongside the weak acid. Equal total concentrations but different ratios affect capacity, with more conjugate base better for acid additions. Capacity isn't just about the ratio but the quantities ready to react. A tempting distractor is that they will change equally because their total buffer concentration is the same, but this is incorrect as Buffer 2's higher conjugate base and 1:1 ratio give it superior capacity. A transferable strategy is, for acid additions, to look for higher concentrations of conjugate base; this means greater capacity to absorb added H⁺.

9

Two buffers are made to the same final volume.

Buffer 1: $\text{HCN}/\text{CN}^-$ with $0.10,\text{mol}$ HCN and $0.10,\text{mol}$ NaCN.

Buffer 2: $\text{HCN}/\text{CN}^-$ with $0.10,\text{mol}$ HCN and $0.020,\text{mol}$ NaCN.

A small, equal amount of strong acid is added to each buffer. Which buffer will show the smaller pH change?

Buffer 2, because a smaller amount of conjugate base makes the buffer stronger

They will change equally because both contain the same moles of HCN

Buffer 2, because it contains more weak acid relative to base

Buffer 1, because it has more conjugate base available to neutralize added acid

They will change equally because HCN is a weak acid in both buffers

Explanation

This question tests your understanding of buffer capacity. Buffer capacity indicates how much acid a buffer can neutralize while keeping pH stable, depending on the total moles of conjugate base available to react with H⁺. More moles of conjugate base mean better resistance without large ratio changes. Capacity relies on these absolute amounts, not merely the ratio. A tempting distractor is that they will change equally because both contain the same moles of HCN, but this ignores Buffer 1's higher conjugate base moles offering better capacity. A transferable strategy is, when adding acid, to check the amount of conjugate base; higher amounts mean greater capacity to absorb added H⁺.

10

A student prepares three buffers, each in a separate beaker, all at the same temperature. Each beaker contains a weak acid and its conjugate base as shown:

  • Buffer 1: a concentrated solution containing equal moles of $\mathrm{HA}$ and $\mathrm{A^-}$
  • Buffer 2: a dilute solution containing equal moles of $\mathrm{HA}$ and $\mathrm{A^-}$
  • Buffer 3: a solution containing much more $\mathrm{HA}$ than $\mathrm{A^-}$

The student adds the same small amount of strong acid, $\mathrm{HCl}$, to each beaker and stirs. Which buffer will resist the pH change the most (have the greatest buffer capacity) for this addition?

Buffer 3, because having excess $\mathrm{HA}$ prevents any change in pH when acid is added

Buffer 2, because equal moles of $\mathrm{HA}$ and $\mathrm{A^-}$ always guarantee the greatest buffer capacity

Buffer 1, because its initial pH must be closer to neutral than the others

Buffer 2, because dilute buffers have fewer ions and therefore change pH less

Buffer 1, because it contains the greatest total amount of buffering components ($\mathrm{HA}$ and $\mathrm{A^-}$)

Explanation

Buffer capacity is the measure of a buffer's ability to resist pH changes upon addition of acid or base. It depends on the total amount of weak acid (HA) and conjugate base (A⁻) available to neutralize added H⁺ or OH⁻, with higher total moles providing greater resistance. For adding acid, the amount of A⁻ is crucial as it reacts with H⁺ to form HA, but overall capacity increases with the total buffering components. The ratio of HA to A⁻ affects the initial pH and the symmetry of capacity, but it's the absolute quantities that determine how much addition can be absorbed. A tempting distractor is that equal moles of HA and A⁻ always guarantee the greatest capacity, but this ignores that dilute solutions with equal ratios have less total material than concentrated ones. To evaluate buffer capacity, compare the total moles of buffering species, as higher total concentration means greater capacity to absorb added acid or base.

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