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Weak Acids and Ka: Flashcards

Revise Ka, pKa, ICE tables and weak acid and base pH.

9 cardsAdvanced

9 cards

  1. Question
    What is a weak acid?
    Answer

    An acid that ionizes only partly in water, setting up an equilibrium:

    HA+HX2O⇌HX3OX++AX−\ce{HA + H2O <=> H3O+ + A-}

  2. Question
    Write the expression for KaK_\text{a}.
    Answer

    Ka=[HX3OX+][AX−][HA]K_\text{a} = \dfrac{[\ce{H3O+}][\ce{A-}]}{[\ce{HA}]}

  3. Question
    Which is the stronger acid: pKa 3.2 or pKa 4.7?
    Answer

    pKa 3.2. A smaller pKa means a larger Ka and a stronger acid.

  4. Question
    What is the shortcut for [HX3OX+][\ce{H3O+}] in a weak acid solution?
    Answer

    [HX3OX+]≈Ka×C[\ce{H3O+}] \approx \sqrt{K_\text{a} \times C}

  5. Question
    When is the shortcut acceptable?
    Answer

    When less than 5% of the acid ionizes (xx < 5% of CC).

  6. Question
    What does ICE stand for?
    Answer

    Initial, Change, Equilibrium.

  7. Question
    What is the pH of 0.20 M acetic acid (Ka=1.8×10−5K_\text{a} = 1.8 \times 10^{-5})?
    Answer

    [HX3OX+]=1.9×10−3[\ce{H3O+}] = 1.9 \times 10^{-3} M, so pH = 2.72

  8. Question
    How are Ka and Kb related for a conjugate pair?
    Answer

    Ka×Kb=Kw=1.0×10−14K_\text{a} \times K_\text{b} = K_\text{w} = 1.0 \times 10^{-14} (at 25 °C)

  9. Question
    Does a weak acid ionize more or less (as a percentage) when diluted?
    Answer

    More. Percent ionization increases on dilution, even though the pH rises.