AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/8/8-10)
Unit 8 · Topic 8.10
8.10 Buffer Capacity
Buffer capacity is how much acid or base a buffer can absorb before its pH changes a lot. A more concentrated buffer has the same pH as a dilute one with the same ratio but more capacity, and a buffer with more acid than base can absorb more added base than added acid.
Key terms
- buffer capacity
- component concentration
- acid-to-base ratio
What capacity means
A buffer works only while it still has enough of both members of its conjugate pair. Each mole of added OH⁻ uses up one mole of the weak acid, and each mole of added H₃O⁺ uses up one mole of the conjugate base. Once one of them runs out, the buffer stops working and the pH changes sharply.
Buffer capacity describes how much acid or base the buffer can absorb before that happens.
Concentration sets capacity, the ratio sets pH
The pH of a buffer depends on the ratio [A⁻]/[HA] (topic 8.9), not on the actual amounts. So 0.10 M CH₃COOH with 0.10 M CH₃COO⁻ and 1.0 M CH₃COOH with 1.0 M CH₃COO⁻ have the same pH, 4.74.
But the concentrated buffer has ten times as many moles of each component, so it can absorb about ten times as much acid or base before one component is used up. Higher concentration of both components (at the same ratio) means greater capacity.
The useful buffering range
Because the pH depends on log([A⁻]/[HA]), a buffer works well only while both components are present in reasonable amounts, roughly when the ratio is between 0.1 and 10. That puts the useful range at about pKa ± 1.
Unequal amounts give unequal capacity
A buffer that is heavy on the weak acid has plenty of HA to neutralize incoming OH⁻ but less A⁻ to neutralize incoming H₃O⁺, so it can take more added base than added acid before it fails. A buffer that is heavy on the conjugate base is the opposite: it handles added acid better.
A buffer with equal amounts of acid and base has the same capacity in both directions. This is one reason buffers are usually made with a ratio near 1.
| Buffer (1.0 L) | pH compared with pKa | Max OH⁻ it can absorb | Max H₃O⁺ it can absorb |
|---|---|---|---|
| 0.10 mol HA + 0.10 mol A⁻ | pH = pKa | 0.10 mol | 0.10 mol |
| 1.0 mol HA + 1.0 mol A⁻ | pH = pKa | 1.0 mol | 1.0 mol |
| 0.30 mol HA + 0.10 mol A⁻ | pH < pKa | 0.30 mol | 0.10 mol |
In the lab
When you test a buffer by adding base drop by drop, the pH stays nearly steady and then shoots up once the weak acid is nearly gone. If a buffer 'fails' sooner than expected, likely causes include components that were more dilute than planned, or a ratio far from 1 so that one component ran out early.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Comparing two buffers
Buffer A is 0.10 M CH₃COOH and 0.10 M NaCH₃COO. Buffer B is 1.0 M CH₃COOH and 1.0 M NaCH₃COO. Compare their pH values and capacities. How many moles of NaOH can 1.0 L of each absorb before the weak acid is used up?
Show the solutionHide the solution
- Step 1: Both have [A⁻]/[HA] = 1, so both have pH = pKa = 4.74. Same pH.
- Step 2: In 1.0 L, Buffer A has 0.10 mol CH₃COOH and Buffer B has 1.0 mol. Each mole of OH⁻ uses up one mole of CH₃COOH.
- Step 3: Buffer A can absorb up to 0.10 mol NaOH; Buffer B up to 1.0 mol. In practice the pH starts climbing steeply as the acid nears zero, so the useful range is a little less than these numbers.
- Step 4: Buffer B has ten times the capacity.
Answer: Same pH (4.74); Buffer B has about ten times the capacity (1.0 mol versus 0.10 mol of NaOH).
- Example 2
Trap: which direction has more capacity?
A buffer contains 0.30 mol HNO₂ and 0.10 mol NaNO₂ in 1.0 L. Is it better at resisting added HCl or added NaOH? A student says HCl, because 'it's an acid buffer'. Evaluate the claim.
Show the solutionHide the solution
- Step 1: Added HCl is neutralized by the conjugate base, NO₂⁻. There's only 0.10 mol of it, so the buffer can absorb at most 0.10 mol HCl.
- Step 2: Added NaOH is neutralized by the weak acid, HNO₂. There's 0.30 mol of it, so the buffer can absorb up to 0.30 mol NaOH.
- Step 3: The student has it backward. More acid component means more capacity against added base.
Answer: It resists added NaOH better (up to 0.30 mol) than added HCl (up to 0.10 mol).
Common mistakes
- Thinking a more concentrated buffer has a different pH. Same ratio means same pH.
- Mixing up directions: more weak acid means more capacity for added base, not added acid.
- Assuming a buffer can absorb any amount of acid or base. It fails once one component is used up.
On the exam
- Expect to compare two buffers' capacities or to say which component limits a buffer against a given addition. Answer in terms of moles of each component available to react.
- Lab-based questions may describe a buffer that failed early and ask why. Point to the amount of the component that reacts with what was added.
Connected topics
Videos
Check yourself
4 questions on 8.10 Buffer Capacity. Pick an answer to see if you got it, and why.
Buffer X is 0.10 M HF and 0.10 M NaF. Buffer Y is 1.0 M HF and 1.0 M NaF. Which of the following correctly compares the two buffers?
A buffer contains 0.40 M HA and 0.10 M A⁻. Which of the following is true about this buffer?
Each of the following buffers is made from the same weak acid, HA, and its conjugate base, A⁻. Which one can neutralize the most added NaOH before its pH rises sharply?
A student needs a buffer with a pH of about 7.2 that can neutralize sizable amounts of both added acid and added base. Which of the following weak acid–conjugate base pairs is the best choice?
0 of 4 answered