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Buffers: Flashcards

Revise how buffers work and the Henderson–Hasselbalch equation.

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9 cards

  1. Question
    What is a buffer?
    Answer

    A solution that resists changes in pH when small amounts of acid or base are added.

  2. Question
    What does a buffer contain?
    Answer

    A weak acid and its conjugate base (or a weak base and its conjugate acid), in similar amounts.

  3. Question
    How does a buffer remove added acid?
    Answer

    The conjugate base reacts with it: AX−+HX3OX+→HA+HX2O\ce{A- + H3O+ -> HA + H2O}

  4. Question
    How does a buffer remove added base?
    Answer

    The weak acid reacts with it: HA+OHX−→AX−+HX2O\ce{HA + OH- -> A- + H2O}

  5. Question
    Write the Henderson–Hasselbalch equation.
    Answer

    pH=pKa+log⁡[AX−][HA]\text{pH} = \text{p}K_\text{a} + \log\dfrac{[\ce{A-}]}{[\ce{HA}]}

  6. Question
    What is the pH of a buffer with equal concentrations of HA and A⁻?
    Answer

    pH = pKa (because log 1 = 0).

  7. Question
    How do you choose a weak acid for a buffer at a particular pH?
    Answer

    Pick one whose pKa is close to (within about 1 unit of) the target pH.

  8. Question
    Is HCl + NaCl a buffer?
    Answer

    No. Cl⁻ is too weak a base to remove added acid. A buffer needs a weak acid and its conjugate base.

  9. Question
    What is buffer capacity?
    Answer

    How much acid or base a buffer can absorb before its pH changes sharply. Higher concentrations give a larger capacity.