AP Chemistry Quiz: Acid Base Reactions And Buffers

What this quiz covers

This quiz focuses on Acid Base Reactions And Buffers, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{HCO_3^-}$ to form $\mathrm{H_2CO_3}$, so the pH decreases slightly. (correct answer)
2. The pH increases because adding $\mathrm{HCl}$ shifts $\mathrm{H_2CO_3}$ to produce more $\mathrm{HCO_3^-}$.
3. The added $\mathrm{H^+}$ reacts mainly with $\mathrm{Na^+}$, so the pH changes very little.
4. The pH decreases sharply because the buffer contains excess weak acid and cannot react with added $\mathrm{H^+}$.
5. The pH remains exactly constant because buffers completely eliminate added $\mathrm{H^+}$.

Explanation: This question tests understanding of properties of buffers. The carbonic acid buffer contains both a weak acid (H₂CO₃) and its conjugate base (HCO₃⁻), though with excess weak acid present. When HCl is added, the H⁺ ions are primarily consumed by the bicarbonate ions (HCO₃⁻) through the reaction: HCO₃⁻ + H⁺ → H₂CO₃. This neutralization prevents most of the added H⁺ from remaining free in solution, resulting in only a slight pH decrease rather than a sharp drop. Option C incorrectly suggests that buffers with excess weak acid cannot react with added H⁺—this misunderstands that it's the conjugate base component that neutralizes added acid. The strategy for buffer problems is to identify which component reacts with the added species: conjugate bases neutralize added acids.

Question 2

1. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{C_2H_3O_2^-}$ to form $\mathrm{HC_2H_3O_2}$, so the pH decreases only slightly. (correct answer)
2. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{HC_2H_3O_2}$ to form $\mathrm{C_2H_3O_2^-}$, so the pH remains exactly constant.
3. The added $\mathrm{H^+}$ remains mostly as free $\mathrm{H^+}$ because buffers neutralize only added base, so the pH decreases sharply.
4. The added $\mathrm{H^+}$ causes $\mathrm{HC_2H_3O_2}$ to dissociate completely, making the solution behave like a strong acid.
5. The added $\mathrm{H^+}$ reacts mainly with $\mathrm{Na^+}$ ions, preventing any change in pH.

Explanation: This question tests understanding of properties of buffers. A buffer solution contains both a weak acid (HC₂H₃O₂) and its conjugate base (C₂H₃O₂⁻), which allows it to resist pH changes when small amounts of acid or base are added. When HCl is added, the H⁺ ions from the strong acid are consumed by the acetate ions (C₂H₃O₂⁻) through the reaction: C₂H₃O₂⁻ + H⁺ → HC₂H₃O₂. This neutralization reaction prevents most of the added H⁺ from remaining free in solution, which would otherwise cause a sharp pH decrease. Option D incorrectly suggests that buffers only neutralize bases, not acids—this is a common misconception since buffers work bidirectionally. The key strategy is to identify which buffer component (the base form) reacts with added acid to minimize pH change.

Question 3

1. The pH decreases sharply because $\mathrm{HA}$ is a weak acid and cannot participate in neutralization reactions.
2. The pH remains exactly constant because the buffer completely prevents any change in $[\mathrm{H_3O^+}]$.
3. The pH decreases only slightly because most added $\mathrm{H^+}$ is consumed by $\mathrm{A^-}$ to form $\mathrm{HA}$. (correct answer)
4. The pH decreases sharply because the presence of excess $\mathrm{A^-}$ increases the concentration of free $\mathrm{H^+}$.
5. The pH increases because added $\mathrm{H^+}$ converts $\mathrm{HA}$ into $\mathrm{A^-}$.

Explanation: This question tests understanding of properties of buffers. The buffer contains a weak acid (HA) and its conjugate base (A⁻), with excess A⁻ present, enabling it to resist pH changes. When HCl is added, the H⁺ ions are consumed by the conjugate base (A⁻) through the reaction: A⁻ + H⁺ → HA. This neutralization prevents most of the added H⁺ from remaining free in solution, resulting in only a slight pH decrease rather than the sharp drop that would occur without the buffer. Option D incorrectly claims that buffers completely prevent any pH change—buffers minimize but don't eliminate pH changes, as the ratio of conjugate base to weak acid does shift slightly. The key strategy is recognizing that conjugate bases in buffers neutralize added acids, converting them to the weak acid form.

Question 4

1. The added $\text{H}^+$ is consumed primarily by $\text{C}_2\text{H}_3\text{O}_2^-$ to form $\text{HC}_2\text{H}_3\text{O}_2$, so the pH decreases only slightly. (correct answer)
2. The added $\text{H}^+$ is consumed primarily by $\text{HC}_2\text{H}_3\text{O}_2$ to form $\text{H}_2\text{C}_2\text{H}_3\text{O}_2^+$, so the pH remains exactly constant.
3. The $\text{HCl}$ converts the buffer into a strong acid solution, causing a large decrease in pH.
4. The $\text{Na}^+$ ions react with the added $\text{H}^+$ to form $\text{NaH}(aq)$, preventing any pH change.
5. The added $\text{H}^+$ reacts primarily with water to form $\text{H}_3\text{O}^+$, so the pH decreases by the same amount as in pure water.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. A common misconception is that buffers keep the pH exactly constant, as in choice B, but actually, small pH changes do occur, though they are minimized. To solve buffer problems, identify which buffer component reacts with the added species—the conjugate base neutralizes added acid, and the weak acid neutralizes added base.

Question 5

1. The pH remains approximately the same because dilution lowers both buffer component concentrations proportionally, leaving their relative amounts unchanged. (correct answer)
2. The pH increases sharply because dilution always makes solutions more basic.
3. The pH decreases sharply because dilution always makes solutions more acidic.
4. The pH becomes 7 because adding water forces neutrality.
5. The pH changes unpredictably because buffers only work at high concentration.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. A common misconception is that dilution changes buffer pH like it does for strong acids, but as in choices B and C, buffer pH is stable due to the maintained ratio. To solve buffer problems, recall that pH depends on the ratio of components, so proportional changes like dilution do not significantly alter pH.

Question 6

1. $\mathrm{HC_2H_3O_2}$ reacts with added $\mathrm{H^+}$ to form $\mathrm{H_2C_2H_3O_2^+}$, preventing any pH change.
2. $\mathrm{C_2H_3O_2^-}$ reacts with added $\mathrm{H^+}$ to form $\mathrm{HC_2H_3O_2}$, removing most of the added $\mathrm{H^+}$. (correct answer)
3. $\mathrm{Na^+}$ reacts with added $\mathrm{H^+}$ to form $\mathrm{NaH(aq)}$, which neutralizes the acid.
4. The added $\mathrm{H^+}$ is consumed mainly by $\mathrm{OH^-}$ already present in the buffer, so the pH stays exactly constant.
5. The added $\mathrm{HCl}$ fully dissociates, so the pH decreases by the same amount as it would in pure water.

Explanation: This question tests understanding of properties of buffers. A buffer contains a weak acid (HC₂H₃O₂) and its conjugate base (C₂H₃O₂⁻) in comparable amounts, which allows it to resist pH changes when small amounts of acid or base are added. When HCl is added, it provides H⁺ ions that are primarily consumed by the conjugate base C₂H₃O₂⁻, forming more of the weak acid HC₂H₃O₂ according to the reaction: C₂H₃O₂⁻ + H⁺ → HC₂H₃O₂. This reaction removes most of the added H⁺ from solution, preventing a large decrease in pH. Choice A is incorrect because HC₂H₃O₂ is already a weak acid and cannot accept another proton to form H₂C₂H₃O₂⁺ under normal conditions. To identify how a buffer responds to added acid or base, determine which buffer component (weak acid or conjugate base) can react with the added species—the conjugate base reacts with added acid, while the weak acid reacts with added base.

Question 7

1. $\mathrm{H_2PO_4^-}$ reacts with the added $\mathrm{H^+}$ to form $\mathrm{HPO_4^{2-}}$, consuming $\mathrm{H^+}$.
2. $\mathrm{HPO_4^{2-}}$ reacts with the added $\mathrm{H^+}$ to form $\mathrm{H_2PO_4^-}$, consuming $\mathrm{H^+}$. (correct answer)
3. $\mathrm{Cl^-}$ reacts with water to form $\mathrm{HClO}$, consuming $\mathrm{H^+}$.
4. Water reacts with the added $\mathrm{H^+}$ to form $\mathrm{OH^-}$, which neutralizes the acid.
5. The added $\mathrm{H^+}$ is consumed primarily by $\mathrm{Na^+}$ impurities in the solution, forming $\mathrm{NaH(aq)}$.

Explanation: This question tests understanding of properties of buffers. The buffer contains the dihydrogen phosphate ion (H₂PO₄⁻) and hydrogen phosphate ion (HPO₄²⁻), which form a conjugate acid-base pair that resists pH changes. When HCl is added, the H⁺ ions react with the more basic species, HPO₄²⁻, to form H₂PO₄⁻ through the reaction: HPO₄²⁻ + H⁺ → H₂PO₄⁻. This reaction consumes the added H⁺ ions, preventing them from significantly lowering the pH of the solution. Choice A is incorrect because it shows H₂PO₄⁻ reacting with H⁺ to form HPO₄²⁻, which would require removing a proton from an already protonated species—this is the opposite of what happens when acid is added. The key strategy is to identify the more basic component in the buffer (the one with fewer protons), as this will be the species that reacts with added acid.

Question 8

1. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{CH_3NH_2}$ to form $\mathrm{CH_3NH_3^+}$, reducing the amount of free $\mathrm{H^+}$. (correct answer)
2. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{Cl^-}$ to form $\mathrm{HCl}$, which is weak.
3. The added $\mathrm{H^+}$ is primarily consumed by $\mathrm{CH_3NH_3^+}$ to form $\mathrm{CH_3NH_4^{2+}}$.
4. The pH does not change because $\mathrm{CH_3NH_2}$ is a strong base that neutralizes all added acid.
5. The pH decreases more than in pure water because the buffer contains additional dissolved ions.

Explanation: This question tests understanding of properties of buffers. The methylamine buffer contains CH₃NH₂ (weak base) and CH₃NH₃⁺ (conjugate acid), allowing it to resist pH changes when small amounts of acid or base are added. When strong acid is added, it provides H⁺ ions that react primarily with the weak base CH₃NH₂ to form CH₃NH₃⁺: CH₃NH₂ + H⁺ → CH₃NH₃⁺. This reaction consumes most of the added H⁺, preventing the large pH decrease that would occur in pure water where all H⁺ remains free in solution. Choice D is incorrect because CH₃NH₂ is a weak base, not a strong base, and buffers do not completely neutralize all added acid—they only minimize pH changes. To understand buffer action, identify which component neutralizes the added species: weak bases consume added acids, while conjugate acids consume added bases.

Question 9

1. Solution 1 shows a smaller pH decrease because weak acids resist pH change better than buffers.
2. Solution 2 shows a smaller pH decrease because $\text{HCOO}^-$ consumes much of the added $\text{H}^+$. (correct answer)
3. Both solutions show the same pH decrease because the same amount of $\text{HCl}$ is added.
4. Solution 2 shows a larger pH decrease because buffers contain more total solute.
5. Neither solution changes pH because $\text{HCl}$ is a strong acid and sets the pH.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. A common misconception is that buffers and weak acids behave the same way, but as shown here, buffers resist pH changes more effectively than weak acids alone, making choice A incorrect. To solve buffer problems, identify which buffer component reacts with the added species—the conjugate base neutralizes added acid, and the weak acid neutralizes added base.

Question 10

1. The added $\mathrm{OH^-}$ is primarily neutralized by $\mathrm{HF}$ to form $\mathrm{F^-}$ and $\mathrm{H_2O}$, so the pH increases only slightly. (correct answer)
2. The added $\mathrm{OH^-}$ is primarily neutralized by $\mathrm{F^-}$ to form $\mathrm{HF}$, so the pH decreases only slightly.
3. The added $\mathrm{OH^-}$ reacts mainly with $\mathrm{Na^+}$, preventing a pH change.
4. The pH remains exactly constant because the buffer converts all added $\mathrm{OH^-}$ into neutral salt with no equilibrium shift.
5. The pH increases sharply because $\mathrm{HF}$ is a weak acid and cannot react with added $\mathrm{OH^-}$.

Explanation: This question tests understanding of properties of buffers. The HF/F⁻ buffer contains both a weak acid (HF) and its conjugate base (F⁻), allowing it to resist pH changes when acids or bases are added. When NaOH is added, the OH⁻ ions are neutralized by the weak acid component (HF) according to: HF + OH⁻ → F⁻ + H₂O. This reaction consumes most of the added hydroxide ions, preventing a sharp pH increase, though the pH does increase slightly as more F⁻ is formed and the ratio of F⁻ to HF increases. Option B incorrectly suggests that F⁻ reacts with OH⁻ to form HF—this is impossible as both F⁻ and OH⁻ are bases and cannot react in this way. The key strategy is to identify that weak acids in buffers neutralize added bases, while conjugate bases neutralize added acids.

Question 11

1. $\mathrm{H_2PO_4^- + H^+ \rightarrow H_3PO_4}$
2. $\mathrm{H^+ + OH^- \rightarrow H_2O}$ (from water only)
3. $\mathrm{HPO_4^{2-} + Cl^- \rightarrow HCl + PO_4^{3-}}$
4. $\mathrm{HPO_4^{2-} + H^+ \rightarrow H_2PO_4^-}$ (correct answer)
5. $\mathrm{H_2PO_4^- + OH^- \rightarrow HPO_4^{2-} + H_2O}$

Explanation: This question tests understanding of properties of buffers. The phosphate buffer contains H₂PO₄⁻ (weak acid) and HPO₄²⁻ (conjugate base), which work together to resist pH changes. When HCl is added, the H⁺ ions are consumed by the conjugate base component (HPO₄²⁻) through the reaction: HPO₄²⁻ + H⁺ → H₂PO₄⁻. This neutralization converts the added strong acid into the weak acid form, preventing a sharp pH decrease and maintaining the buffer's effectiveness. Option C shows H₂PO₄⁻ accepting another proton, but this is not the primary buffering reaction since HPO₄²⁻ is more basic and reacts preferentially with added H⁺. The strategy for identifying buffer reactions is to recognize that the more basic component (higher charge on phosphate) neutralizes added acid.

Question 12

1. The pH decreases sharply because adding $\mathrm{OH^-}$ increases $[\mathrm{H_3O^+}]$ through water autoionization.
2. The pH decreases slightly because $\mathrm{OH^-}$ reacts with $\mathrm{B}$ to form $\mathrm{BH^+}$.
3. The pH remains exactly constant because $\mathrm{BH^+}$ and $\mathrm{B}$ are both weak and therefore unreactive.
4. The pH increases sharply because the buffer contains mostly $\mathrm{BH^+}$ and cannot react with $\mathrm{OH^-}$.
5. The pH increases only slightly because $\mathrm{OH^-}$ is consumed by $\mathrm{BH^+}$ to form $\mathrm{B}$ and $\mathrm{H_2O}$. (correct answer)

Explanation: This question tests understanding of properties of buffers. The buffer contains a weak base (B) and its conjugate acid (BH⁺), with excess BH⁺ present, allowing it to resist pH changes. When NaOH is added, the OH⁻ ions are consumed by the conjugate acid component (BH⁺) according to: BH⁺ + OH⁻ → B + H₂O. This neutralization reaction prevents most of the added hydroxide from remaining free in solution, resulting in only a slight pH increase rather than a sharp rise. Option A incorrectly suggests that OH⁻ reacts with B to form BH⁺—this is impossible as bases cannot react with other bases to form acids without a proton source. The strategy for buffer problems is to identify that conjugate acids neutralize added bases, while weak bases neutralize added acids.

Question 13

1. The claim is correct; buffers maintain a perfectly constant pH regardless of what is added.
2. The claim is incorrect; a buffer contains no species that can react with added $\mathrm{OH^-}$.
3. The claim is incorrect; added $\mathrm{OH^-}$ reacts with $\mathrm{HA}$ to form $\mathrm{A^-}$, so pH changes slightly rather than not at all. (correct answer)
4. The claim is correct; added $\mathrm{OH^-}$ reacts primarily with $\mathrm{A^-}$ to form $\mathrm{HA}$, leaving pH unchanged.
5. The claim is incorrect; buffers work only when strong acids are added, not when strong bases are added.

Explanation: This question tests understanding of properties of buffers. The student's claim that pH will not change at all is incorrect because buffers minimize but do not completely eliminate pH changes. When a strong base like NaOH is added to an HA/A⁻ buffer, the OH⁻ ions react with the weak acid component: HA + OH⁻ → A⁻ + H₂O. This reaction consumes most of the added base and converts HA to A⁻, causing the ratio of conjugate base to weak acid to increase, which results in a slight pH increase. Option B incorrectly states that buffers contain no species that can react with OH⁻—the weak acid component specifically serves this purpose. The key concept is that buffers resist but don't prevent pH changes, and the strategy is to identify which component (weak acid) neutralizes added base.

Question 14

1. The pH decreases because adding $\text{CH}_3\text{COO}^-$ produces more $\text{H}_3\text{O}^+$.
2. The pH increases because adding $\text{CH}_3\text{COO}^-$ increases the solution's ability to consume $\text{H}^+$, shifting the acid–base equilibrium toward fewer hydronium ions. (correct answer)
3. The pH remains exactly constant because buffers cannot change pH under any circumstance.
4. The pH becomes 7 because sodium salts make solutions neutral.
5. The pH decreases because acetate is a strong acid.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. A common misconception is that adding more conjugate base decreases pH, as in choice A, but it actually increases pH by shifting the equilibrium. To solve buffer problems, identify which buffer component reacts with the added species or how changes in ratio affect pH using the Henderson-Hasselbalch equation.

Question 15

1. The added $\text{OH}^-$ is neutralized primarily by $\text{NH}_3$ to form $\text{NH}_2^-$, so the pH decreases slightly.
2. The added $\text{OH}^-$ reacts mainly with water, so the pH rises as much as it would in pure water.
3. Because it is a buffer, the pH does not change at all when $\text{NaOH}$ is added.
4. The $\text{Cl}^-$ ions react with $\text{OH}^-$ to form $\text{HOCl}$, keeping the pH constant.
5. The added $\text{OH}^-$ is neutralized primarily by $\text{NH}_4^+$ to form $\text{NH}_3$ and $\text{H}_2\text{O}$, so the pH increases only slightly. (correct answer)

Explanation: This question assesses the properties of buffers. Buffers contain a weak base and its conjugate acid, which work together to resist pH changes. When base is added, the conjugate acid reacts with the added OH⁻ to form more weak base and water, consuming the OH⁻ and preventing a large rise in pH. When acid is added, the weak base reacts with the added H⁺ to form more conjugate acid, consuming the H⁺ and preventing a large drop in pH. A common misconception is that buffers prevent any pH change at all, as in choice D, but in reality, they only minimize the change, allowing a slight increase or decrease. To solve buffer problems, identify which buffer component reacts with the added species—the conjugate acid neutralizes added base, and the weak base neutralizes added acid.

Question 16

1. The pH increases only slightly because $\text{CH}_3\text{NH}_3^+$ neutralizes added $\text{OH}^-$ to form $\text{CH}_3\text{NH}_2$ and $\text{H}_2\text{O}$. (correct answer)
2. The pH decreases because added $\text{OH}^-$ shifts the buffer equilibrium to produce more $\text{H}_3\text{O}^+$.
3. The pH remains exactly constant because buffers completely eliminate added $\text{OH}^-$.
4. The pH increases greatly because the buffer contains a weak base, so it cannot react with $\text{OH}^-$.
5. The pH decreases greatly because $\text{CH}_3\text{NH}_3^+$ is a strong acid that reacts completely with water.

Explanation: This question assesses the properties of buffers. Buffers contain a weak base and its conjugate acid, which work together to resist pH changes. When base is added, the conjugate acid reacts with the added OH⁻ to form more weak base and water, consuming the OH⁻ and preventing a large rise in pH. When acid is added, the weak base reacts with the added H⁺ to form more conjugate acid, consuming the H⁺ and preventing a large drop in pH. A common misconception is that buffers eliminate added species completely without any pH change, as in choice C, but they only minimize the change, not prevent it entirely. To solve buffer problems, identify which buffer component reacts with the added species—the conjugate acid neutralizes added base, and the weak base neutralizes added acid.

Question 17

1. Both $\text{HCN}$ and $\text{CN}^-$ increase because adding base shifts all equilibria to the right.
2. $\text{HCN}$ decreases and $\text{CN}^-$ increases because $\text{HCN}$ neutralizes added $\text{OH}^-$. (correct answer)
3. $\text{HCN}$ increases and $\text{CN}^-$ decreases because $\text{CN}^-$ neutralizes added $\text{OH}^-$.
4. Neither changes because buffers keep the concentrations of all species constant.
5. $\text{CN}^-$ is converted to $\text{CNO}^-$ because strong base oxidizes cyanide.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. A common misconception is that buffer components remain unchanged in concentration, as in choice D, but actually, the reacting component decreases while the other increases. To solve buffer problems, identify which buffer component reacts with the added species—the weak acid neutralizes added base, and the conjugate base neutralizes added acid.

Question 18

1. $\text{NH}_4^+$, because it accepts $\text{H}^+$ to form $\text{NH}_5^{2+}$.
2. $\text{NH}_3$, because it accepts $\text{H}^+$ to form $\text{NH}_4^+$. (correct answer)
3. $\text{Cl}^-$ (from the acid), because it accepts $\text{H}^+$ to form $\text{HCl}$.
4. Water, because buffers do not react with added acid.
5. $\text{NH}_4^+$, because it donates $\text{OH}^-$ to neutralize $\text{H}^+$.

Explanation: This question assesses the properties of buffers. Buffers contain a weak base and its conjugate acid, which work together to resist pH changes. When acid is added, the weak base reacts with the added H⁺ to form more conjugate acid, consuming the H⁺ and preventing a large drop in pH. When base is added, the conjugate acid reacts with the added OH⁻ to form more weak base and water, consuming the OH⁻ and preventing a large rise in pH. A common misconception is that the conjugate acid reacts directly with added acid, as in choice A, but actually, the base component handles added acid. To solve buffer problems, identify which buffer component reacts with the added species—the weak base neutralizes added acid, and the conjugate acid neutralizes added base.

Question 19

1. The added $\text{OH}^-$ is converted to $\text{O}_2(g)$ by $\text{NO}_2^-$, so pH cannot rise.
2. The added $\text{OH}^-$ reacts with $\text{HNO}_2$ to form $\text{NO}_2^-$ and water, reducing the amount of free $\text{OH}^-$. (correct answer)
3. The added $\text{OH}^-$ is neutralized by $\text{Na}^+$ to form $\text{NaOH}(l)$, removing base from solution.
4. The buffer works by preventing any reactions from occurring, so $\text{OH}^-$ remains unreacted but pH stays constant.
5. The pH increase is limited because $\text{HNO}_2$ is a strong acid and fixes the pH.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. A common misconception is that buffers prevent reactions from happening, as in choice D, but they actually promote specific reactions to stabilize pH. To solve buffer problems, identify which buffer component reacts with the added species—the weak acid neutralizes added base, and the conjugate base neutralizes added acid.

Question 20

1. $\text{NO}_2^-$ reacts with $\text{OH}^-$ to form $\text{NO}_2\text{OH}^{2-}$, which keeps the pH constant.
2. $\text{HNO}_2$ reacts with $\text{OH}^-$ to form $\text{NO}_2^-$ and $\text{H}_2\text{O}$, limiting the pH increase. (correct answer)
3. $\text{Na}^+$ reacts with $\text{OH}^-$ to form $\text{NaOH}_2^+$, consuming the base.
4. $\text{NO}_2^-$ reacts with water to form $\text{HNO}_2$ and $\text{OH}^-$, so adding $\text{OH}^-$ lowers the pH.
5. $\text{OH}^-$ reacts primarily with water, so the buffer components are not involved.

Explanation: This question assesses the properties of buffers. Buffers contain a weak acid and its conjugate base, which work together to resist pH changes. When base is added, the weak acid reacts with the added OH⁻ to form more conjugate base and water, consuming the OH⁻ and preventing a large rise in pH. When acid is added, the conjugate base reacts with the added H⁺ to form more weak acid, consuming the H⁺ and preventing a large drop in pH. A common misconception is that buffers keep pH constant by preventing reactions, as in choice E, but actually, they rely on reactions between components and added species. To solve buffer problems, identify which buffer component reacts with the added species—the weak acid neutralizes added base, and the conjugate base neutralizes added acid.