Buffer Capacity
Separate ratio-controlled pH from total and directional capacity; compare controlled equal-volume samples.
AP Chemistry Topic 8.10 · CED printed p. 158 / PDF p. 165. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Buffer reactions
- Ratio versus absolute concentration
- Experimental controls
Two buffers can have the same pH and very different staying power. pH depends mainly on the ratio of conjugate forms. Capacity depends on how much of each form is available to react. Multiply both component concentrations by the same factor: their ratio and approximate pH remain the same, but the solution can neutralize more added acid or base per liter. For whole samples, also compare volume; twice the volume at the same concentrations contains twice the reservoir moles. A large acid/base addition can overwhelm even a buffer whose original pH exactly equals pKa.
Capacity can be directional. More HA than A− gives a larger reserve to consume added base, because HA donates the needed protons. More A− gives a larger reserve to consume added acid, because A− accepts its protons. A pair near a 1:1 ratio offers balanced protection in both directions, but that does not guarantee enough total amount. In a supplied-data investigation, compare equal sample volumes, equal added moles, the same pair and temperature, and a consistent criterion for “large pH change.” Unexpected dilution may leave initial pH nearly unchanged while making the measured sample less resistant. Explain an error by tracing which component reservoir the procedure actually changes.
Let's work through it
- Two 50.0 mL samples have pKa = 4.80 and [A−]/[HA] = 2.00. Sample I has [HA] = 0.100 M; II has [HA] = 0.0200 M.
- Their current pH is the same, pKa + log 2.00. In I, [A−] = 0.200 M; in II, [A−] = 0.0400 M.
- I contains five times the moles of each buffer component and therefore has substantially more capacity for either type of addition.
- Both have more A− than HA, so each has a larger reserve for added acid than for added base. Equal pH did not determine either total or directional capacity.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Which buffer component determines the reserve against added acid?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Which buffer component determines the reserve against added acid?
Worked answer and reason
The conjugate base, which accepts the added proton.
Capacity follows the reacting reservoir.
Retrieval 2
Why can diluted buffer retain its pH but lose capacity?
Worked answer and reason
Both forms dilute proportionally while their available moles per test volume decrease.
Ratio and reservoir size are distinct quantities.
Catch the tempting mistake
- The lowest-pH buffer always has the greatest capacity.
- Capacity requires actual component amounts and the direction of the challenge, not pH alone.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 8 problem 14 · ap application · data
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For the teacher
- Use identical starting-pH buffers in an error-analysis task. Students should not calculate excluded numerical buffer pH changes.
- The shared acidbase model illustrates fixed-concentration titration and pair-ratio context; it does not vary component concentration and is not evidence for capacity scaling.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 8.10.A
- Explain the relationship between the buffer capacity of a solution and the relative concentrations of the conjugate acid and conjugate base components of the solution.
- 8.10.A.1
- Increasing the concentration of the buffer components (while keeping the ratio of these concentrations constant) keeps the pH of the buffer the same but increases the capacity of the buffer to neutralize added acid or base.
- 8.10.A.2
- When a buffer has more conjugate acid than base, it has a greater buffer capacity for addition of added base than acid. When a buffer has more conjugate base than acid, it has a greater buffer capacity for addition of added acid than base.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §14.6, Buffers — Conjugate-pair buffer reactions, composition ratio, and capacity.
Glossary and deeper reading · 4 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Buffer
A buffer contains appreciable amounts of a weak acid and its conjugate base, or the corresponding weak-base pair. Those partners consume small added amounts of strong base or acid, limiting the pH change within the buffer's capacity. For example, an acetic acid–acetate mixture can resist modest additions of acid or base.
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pH
pH is the negative base-ten logarithm of hydronium activity. In suitable dilute models, activity is approximated by molar concentration relative to the standard concentration. For example, lowering hydronium concentration by a factor of ten raises pH by one unit.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Dilution
Dilution lowers solute concentration by adding solvent while conserving the solute amount when no reaction or loss occurs. Concentration calculations use the final solution volume rather than simply the volume of solvent added. For example, increasing a solution's volume from 100 to 200 milliliters halves its concentration if its solute amount is unchanged.