Buffer Capacity - AP Chemistry

1. Two buffers are prepared using equal volumes at the same temperature. Buffer A: $\mathrm{H_3PO_4/H_2PO_4^-}$ with relatively large amounts of both components Buffer B: $\mathrm{H_3PO_4/H_2PO_4^-}$ with relatively small amounts of both components The same small amount of strong base is added to each buffer. Which statement best explains which buffer has greater capacity?
   Answer: Buffer A has greater capacity because it contains more total moles of the weak acid and its conjugate base to neutralize added base. This question evaluates buffer capacity principles. Buffer capacity is determined by the total moles of weak acid and conjugate base available to neutralize added H⁺ or OH⁻, not by solution volume or other factors. Buffer A contains relatively large amounts of both H₃PO₄ and H₂PO₄⁻, while Buffer B has relatively small amounts of both. When strong base is added, the H₃PO₄ (weak acid) will react to neutralize the OH⁻ ions, and Buffer A has more moles of H₃PO₄ available for this reaction. The misconception in choice A is that smaller amounts somehow change pH less, but this confuses dilution effects with buffer capacity. The key principle is that higher total concentration of buffer components provides greater capacity to resist pH changes from added acid or base.
2. A student prepares two different buffer solutions, each in a final volume of $1.0\,\text{L}$: - Buffer A: $0.50\,\text{mol}$ HNO$_2$ and $0.50\,\text{mol}$ NaNO$_2$ - Buffer B: $0.50\,\text{mol}$ NH$_3$ and $0.50\,\text{mol}$ NH$_4$Cl The same small amount of strong acid (HCl) is added to each buffer. Which statement best compares the buffer capacities of A and B under these conditions?
   Answer: They have approximately the same buffer capacity because each contains the same total amount of conjugate pair available to react with added H$^+$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of weak acid and conjugate base available to neutralize added H⁺ or OH⁻, not just their ratio or the identity of the buffer system. When HCl is added to Buffer A, NO₂⁻ reacts with H⁺; when HCl is added to Buffer B, NH₃ reacts with H⁺. Both buffers contain 0.50 mol of the species that neutralizes H⁺ (NO₂⁻ in A, NH₃ in B), giving them approximately equal capacity to resist pH change from the same amount of added acid. The distractor about weak acids buffering better (option A) is incorrect because capacity depends on the amount of neutralizing species, not whether the buffer is acid/base or base/acid type. The key strategy is that higher total concentration of buffer components means greater capacity to absorb added acid or base.
3. 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?
   Answer: Buffer 1, because it has more conjugate base available to neutralize added acid 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⁺.
4. A student prepares two phosphate buffers at the same temperature, each in a final volume of $1.0\,\text{L}$: - Buffer 1: $0.20\,\text{mol}$ H$_2$PO$_4^-$ (from NaH$_2$PO$_4$) and $0.20\,\text{mol}$ HPO$_4^{2-}$ (from Na$_2$HPO$_4$) - Buffer 2: $0.60\,\text{mol}$ H$_2$PO$_4^-$ and $0.10\,\text{mol}$ HPO$_4^{2-}$ The same small amount of strong acid (HCl) is added to each buffer. Which buffer will best resist the pH change caused by the added HCl?
   Answer: Buffer 1, because it contains more HPO$_4^{2-}$ available to react with added H$^+$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of weak acid and conjugate base available to neutralize added H⁺ or OH⁻, not just their ratio. When HCl is added, the conjugate base (HPO₄²⁻) reacts with H⁺ to form H₂PO₄⁻, so we need sufficient HPO₄²⁻ to neutralize the added acid. Buffer 1 has 0.20 mol HPO₄²⁻ while Buffer 2 has only 0.10 mol HPO₄²⁻, making Buffer 1 twice as capable of resisting pH change. The distractor about more total phosphate (option A) is incorrect because it's specifically the amount of conjugate base that matters for neutralizing added acid. The key strategy is that higher total concentration of buffer components means greater capacity to absorb added acid or base.
5. 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?
   Answer: Buffer 1, because it contains the greatest total amount of buffering components ($\mathrm{HA}$ and $\mathrm{A^-}$) 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.
6. A teacher sets up two buffer solutions of equal volume using the same weak acid/conjugate base pair, then adds the same small amount of strong base to each. - Buffer 1 contains a large amount of $\mathrm{HA}$ and a small amount of $\mathrm{A^-}$. - Buffer 2 contains a large amount of $\mathrm{A^-}$ and a small amount of $\mathrm{HA}$. Which buffer will better resist the pH change caused by adding strong base?
   Answer: Buffer 1, because it contains more $\mathrm{HA}$ to react with and consume added $\mathrm{OH^-}$ Buffer capacity is the ability of a buffer to maintain pH stability against added acid or base. It is based on the total amount of HA and A⁻ present to neutralize OH⁻ or H⁺, with more of the appropriate component enhancing resistance. For base addition, HA reacts with OH⁻, so buffers with abundant HA perform better. Ratios determine initial pH, but total moles define capacity limits. A tempting distractor is that more conjugate base resists base better, but actually, the weak acid is crucial for neutralizing added base. To optimize for additions, ensure high amounts of the neutralizing component, as this boosts capacity to handle acid or base.
7. Two buffers are prepared with the same conjugate pair and the same total volume. Buffer 1: $\text{H}_3\text{PO}_4/\text{H}_2\text{PO}_4^-$ with $0.50\,\text{M}$ $\text{H}_3\text{PO}_4$ and $0.50\,\text{M}$ $\text{NaH}_2\text{PO}_4$. Buffer 2: $\text{H}_3\text{PO}_4/\text{H}_2\text{PO}_4^-$ with $0.10\,\text{M}$ $\text{H}_3\text{PO}_4$ and $0.10\,\text{M}$ $\text{NaH}_2\text{PO}_4$. The same small amount of strong base is added to each. Which buffer will experience the smaller pH change?
   Answer: Buffer 1 This question tests your understanding of buffer capacity. Buffer capacity measures a buffer's ability to resist pH shifts when adding base, depending on the total concentrations of weak acid and conjugate base to neutralize OH⁻. Greater concentrations allow more neutralization with less impact on the ratio. It's the overall levels, not just the ratio, that determine this resistance. A tempting distractor is that they will change equally because the acid-to-base ratio is the same, but this is incorrect because Buffer 1's higher concentrations provide superior capacity. A transferable strategy is to evaluate total buffer concentrations; higher total concentration means greater capacity to absorb added acid or base.
8. 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?
   Answer: Buffer B, because it contains a greater amount of both buffer components 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.
9. Two buffers are prepared using the same conjugate acid–base pair, $\mathrm{NH_4^+/NH_3}$. Both buffers have the same $\mathrm{NH_4^+ : NH_3}$ ratio, but Buffer X is made by dissolving much larger amounts of both components in the same final volume than Buffer Y. The same small amount of strong base, $\mathrm{NaOH}$, is added to equal volumes of Buffer X and Buffer Y. Which statement best compares the buffers' ability to resist a pH change?
   Answer: Buffer X resists the pH change more because it contains more total moles of $\mathrm{NH_4^+}$ and $\mathrm{NH_3}$ available to neutralize added base Buffer capacity describes how well a buffer maintains its pH when acid or base is added. It relies on the total amount of conjugate acid and base present to neutralize the added species, not merely their ratio. Buffers with more total moles can handle larger additions of H⁺ or OH⁻ before significant pH shifts occur. While the ratio determines the initial pH, capacity increases with higher concentrations of both components. A common misconception is that buffers with the same ratio have equal capacity, but this overlooks that lower total amounts limit the neutralization potential. When comparing buffers, prioritize the one with higher total moles of buffering components, as this enhances capacity to resist pH changes from added acid or base.
10. Two buffers are prepared using weak bases and their conjugate acids: - Buffer X: $\mathrm{B/BH^+}$ with a high concentration of both components - Buffer Y: $\mathrm{B/BH^+}$ with a low concentration of both components Equal volumes of each are treated with the same small amount of strong acid. Which statement correctly compares their buffer capacities?
    Answer: Buffer X has greater capacity because it contains more total moles of $\mathrm{B}$ available to neutralize the added $\mathrm{H^+}$ Buffer capacity indicates how effectively a buffer resists pH variations from acid or base addition. It depends on the total quantities of base and conjugate acid available to counteract H⁺ or OH⁻, not solely the pair's identity. High-concentration buffers have more moles to neutralize additions in equal volumes. The ratio influences pH, but total amount is key for capacity. A common misconception is that same conjugate pairs yield equal capacity, but differences in total concentration affect performance. For capacity comparisons, prioritize buffers with higher total moles, as this increases resistance to pH changes from added acid or base.
11. A student prepares two different buffers, each with total volume 1.00 L, and then adds the same small amount of strong base to each. - Buffer 1 (H$_2$CO$_3$/HCO$_3^-$): 0.15 mol H$_2$CO$_3$ and 0.15 mol HCO$_3^-$ - Buffer 2 (HClO/ClO$^-$): 0.30 mol HClO and 0.30 mol ClO$^-$ Which buffer will have the greater capacity to resist the pH increase caused by the added base?
    Answer: Buffer 2, because it contains more total moles of the conjugate acid–base pair available to neutralize added OH$^-$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of acid and conjugate base available to neutralize added H⁺ or OH⁻, not on whether the system is polyprotic or the specific identity of the acid. Buffer 2 contains 0.30 mol each of HClO and ClO⁻ (0.60 mol total), while Buffer 1 contains only 0.15 mol each of H₂CO₃ and HCO₃⁻ (0.30 mol total). When strong base is added, it reacts with the weak acid component, and Buffer 2 has twice as much weak acid available to neutralize the OH⁻. The misconception in choice A is that polyprotic acids automatically provide better buffering, but what matters is the total moles available. Higher total concentration means greater capacity to absorb added acid or base.
12. 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?
    Answer: Buffer B, because it contains more moles of both HA and $A^{-}$ per liter 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.
13. Two buffers are prepared at the same temperature, each in a total volume of 1.00 L, and then the same small amount of strong acid is added to each. - Buffer A: 0.10 mol CH$_3$NH$_2$ and 0.90 mol CH$_3$NH$_3^+$ - Buffer B: 0.90 mol CH$_3$NH$_2$ and 0.10 mol CH$_3$NH$_3^+$ Which buffer will better resist the pH decrease caused by the added strong acid?​​
    Answer: Buffer B, because it contains more weak base to react with and consume added H$^+$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of acid and conjugate base available to neutralize added H⁺ or OH⁻, with the specific component that matters depending on what is being added. When strong acid is added to a buffer, it reacts with the weak base component (CH₃NH₂ in this case). Buffer B contains 0.90 mol CH₃NH₂ while Buffer A contains only 0.10 mol CH₃NH₂, so Buffer B has nine times more base available to neutralize the added H⁺. The misconception in choice A is thinking that the conjugate acid (CH₃NH₃⁺) neutralizes H⁺, but it cannot accept additional protons. When adding acid to a buffer, the amount of weak base present determines the buffer's capacity to resist pH change.
14. A student compares two buffers made from different conjugate pairs but prepared so that each contains equal concentrations of the weak species and its conjugate. Buffer A: $\text{HNO}_2/\text{NO}_2^-$ with $0.30\,\text{M}$ $\text{HNO}_2$ and $0.30\,\text{M}$ $\text{NaNO}_2$. Buffer B: $\text{HOCl}/\text{OCl}^-$ with $0.60\,\text{M}$ HOCl and $0.60\,\text{M}$ NaOCl. The same small amount of strong acid is added to each. Which buffer is expected to have greater buffer capacity?
    Answer: Buffer B, because it has higher concentrations of both buffer components This question tests your understanding of buffer capacity. Buffer capacity assesses resistance to pH change from added acid, based on total concentrations of weak acid and conjugate base available for neutralization. Higher concentrations provide more components to react, minimizing ratio shifts. Different pairs can be compared by their concentrations, not just Ka values. A tempting distractor is that they have equal capacity because both are prepared with equal acid and base concentrations, but this is wrong as Buffer B's higher concentrations yield greater capacity. A transferable strategy is to compare total concentrations regardless of the specific pair; higher total concentration means greater capacity to absorb added acid or base.
15. Two buffers are prepared in separate beakers, each with total volume 1.00 L. - Buffer P: 0.60 mol HCOOH and 0.20 mol HCOO$^-$ - Buffer Q: 0.20 mol HCOOH and 0.60 mol HCOO$^-$ The same small amount of strong base (NaOH) is added to each buffer. Which buffer will undergo the smaller pH change?
    Answer: Buffer P, because it has more weak acid available to react with added OH$^-$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of acid and conjugate base available to neutralize added H⁺ or OH⁻, with the specific component that matters depending on what is being added. When strong base (NaOH) is added to a buffer, it reacts with the weak acid component (HCOOH in this case). Buffer P contains 0.60 mol HCOOH while Buffer Q contains only 0.20 mol HCOOH, so Buffer P has three times more acid available to neutralize the added OH⁻. The misconception in choice C is thinking that more conjugate base always means greater capacity, but this depends on whether acid or base is being added. When adding base to a buffer, focus on the amount of weak acid available to react with it.
16. 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?
    Answer: Buffer A, because it contains more weak acid to neutralize added base 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⁻.
17. 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?
    Answer: Buffer N has greater capacity because it contains more total buffering species to convert added $\mathrm{H^+}$ into $\mathrm{HA}$ 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.
18. A student prepares two buffers, each in a $1.0\,\mathrm{L}$ flask: - Buffer A: $\mathrm{HA}$ and $\mathrm{A^-}$ are present in equal amounts, but the solution is dilute. - Buffer B: $\mathrm{HA}$ and $\mathrm{A^-}$ are present in unequal amounts (more $\mathrm{A^-}$ than $\mathrm{HA}$), but the total amount of $\mathrm{HA}+\mathrm{A^-}$ is much larger than in Buffer A. The same small amount of strong acid is added to each buffer. Which buffer is expected to show the smaller pH change overall?
    Answer: Buffer B, because the larger total amount of buffering species provides greater capacity to neutralize added acid Buffer capacity is the buffer's capability to withstand pH changes when acid or base is added. It relies on the total amount of HA and A⁻ to neutralize additions, with larger totals offering more protection. Even if ratios differ, higher overall moles can provide superior capacity. The ratio sets the pH, but absolute quantities control the extent of resistance. A tempting distractor is that equal amounts maximize capacity regardless of total, but unequal ratios with higher totals can outperform dilute equal ones. To determine superior capacity, look at total buffering moles, as greater totals mean enhanced ability to absorb added acid or base.
19. Two buffers are prepared using the same weak base/conjugate acid pair. Buffer 1: $\text{B}/\text{BH}^+$ with $0.60\,\text{M}$ B and $0.20\,\text{M}$ BH$^+$. Buffer 2: $\text{B}/\text{BH}^+$ with $0.20\,\text{M}$ B and $0.60\,\text{M}$ BH$^+$. The same small amount of strong acid is added to each buffer. Which buffer will show the smaller pH change?
    Answer: Buffer 1, because it contains more weak base to neutralize added acid This question tests your understanding of buffer capacity. Buffer capacity indicates resistance to added acid, depending on the total concentrations of weak base available to neutralize H⁺. Higher weak base concentrations better handle acid without major shifts. It's the amounts, not just ratio, that count. A tempting distractor is that they will change equally because the total buffer concentration is the same, but this ignores Buffer 1's higher weak base better suiting acid addition. A transferable strategy is, for acid additions to basic buffers, to look for higher weak base concentration; this means greater capacity to absorb added H⁺.
20. Two buffers are prepared using the same weak acid/conjugate base pair, HNO$_2$/NO$_2^-$. Each buffer is made by mixing solutions to a total volume of 1.00 L. - Buffer S: 0.050 mol HNO$_2$ and 0.150 mol NO$_2^-$ - Buffer T: 0.50 mol HNO$_2$ and 1.50 mol NO$_2^-$ The same small amount of strong acid is added to both buffers. Which statement best compares the buffer capacities of S and T for this addition?
    Answer: Buffer T has greater capacity because it contains more total moles of buffering components to react with added H$^+$ This question tests understanding of buffer capacity. Buffer capacity depends on the total amount of acid and conjugate base available to neutralize added H⁺ or OH⁻, not just the ratio of components. Buffer T contains 0.50 mol HNO₂ and 1.50 mol NO₂⁻ (2.0 mol total), while Buffer S contains only 0.050 mol HNO₂ and 0.150 mol NO₂⁻ (0.20 mol total). When strong acid is added, it reacts with the conjugate base NO₂⁻, and Buffer T has ten times more conjugate base available to neutralize the added H⁺. The misconception in choice D is that having the same ratio (1:3 in both buffers) means equal capacity, but this ignores the importance of total amount. To determine buffer capacity, always compare the total moles of buffering components available to react with added acid or base.

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