U8.10 · Buffer CapacityPractice · 缓冲容量
Lessons covered:U8.8 Properties of BuffersU8.10 Buffer Capacity
- 1Multiple choice
What mainly determines a buffer’s capacity?
ExplanationCapacity is how much acid or base can be neutralized before the pH changes noticeably. The weak acid neutralizes added base and the conjugate base neutralizes added acid, so it comes down to their moles. Two buffers with the same pH, Ka, and ratio differ in capacity if one is more concentrated or larger in volume. The ratio affects whether the capacity is larger toward acid or toward base.
Review: U8.10 Buffer Capacity
- 2Numeric
1.00 L of a buffer contains 0.25 mol each of CH₃COOH and CH₃COONa (pKa = 4.74). What is the most NaOH, in moles, that can be added (ignore the volume change) before the pH rises by 1?
molExplanationThe starting pH is the pKa. For the pH to rise by 1, [A⁻]/[HA] must reach 10. Adding x mol OH⁻: (0.25 + x) ÷ (0.25 − x) = 10, so 0.25 + x = 2.5 − 10x, 11x = 2.25, and x = 0.205 mol.
Review: U8.10 Buffer Capacity
- 3Multiple choice
A buffer contains 0.40 mol of a weak acid HA and 0.10 mol of its conjugate base A⁻. Which statement about its capacity is correct?
ExplanationAdded base is neutralized by HA (HA + OH⁻ → A⁻ + H₂O), of which there is 0.40 mol; added acid is neutralized by A⁻, of which there is only 0.10 mol. So it resists base better. A 4 : 1 ratio is still within 0.1–10, so it is a working buffer.
Review: U8.10 Buffer Capacity
- 4Numeric
200.0 mL of a buffer is 0.050 M in both CH₃COOH and CH₃COO⁻. What is the pH after 12.0 mmol NaOH is added (ignore the volume change)?
ExplanationThere is only 200.0 × 0.050 = 10.0 mmol of CH₃COOH. The 12.0 mmol of OH⁻ neutralizes all of it with 2.0 mmol to spare, so the buffer is exhausted. [OH⁻] = 2.0 ÷ 200.0 = 0.010 M and pH = 12.00 (the CH₃COO⁻ contributes negligibly).
Review: U8.10 Buffer Capacity
- 5Multiple choice
Roughly what pH range can a buffer made from a weak acid and its conjugate base handle effectively?
ExplanationpH = pKa ± 1 corresponds to [A⁻]/[HA] between 0.1 and 10, so both components are present in useful amounts and both acid and base are resisted. Outside that range one component is too scarce to buffer in one direction. Higher concentration increases capacity but doesn’t widen the effective pH range.
Review: U8.10 Buffer Capacity
- 6Multiple choice
Two acetic acid–sodium acetate buffers are each 1.00 L: buffer 1 contains 0.25 mol each of CH₃COOH and CH₃COONa, and buffer 2 contains 0.025 mol of each. HCl is added gradually to each, and the pH changes as shown. Which buffer has the greater buffer capacity, and how does the graph show it?
ExplanationBoth have [A⁻]/[HA] = 1, so both start at pH 4.74 (= pKa). After 0.020 mol of HCl, buffer 1 is at about pH 4.68, almost unchanged, while buffer 2 is at about 3.81; near 0.025 mol its acetate runs out and the pH plunges. Buffer capacity is how much acid (or base) a buffer can absorb without a large pH change, and it depends on the moles of the two components, not just their ratio and the pKa.
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