Weak Acids, Ka and pKa · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Weak acids, Ka and pKa

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. Acid X has pKa 3 and acid Y has pKa 5. Which is stronger?

    • Y, since higher pKa wins
    • They are equally strong
    • X, since lower pKa wins

  2. Which Ka expression fits the weak acid HA?

    • [H+][A-]/[HA]
    • [HA]/[H+][A-]
    • [H+][HA]/[A-]

  3. Equal volumes of 0.10 M ethanoic acid and 0.10 M HCl need the same volume of the same alkali to neutralise.

    Circle one:   True   False

  4. Why is the pH of 0.10 M ethanoic acid higher than the pH of 0.10 M HCl?

    • The weak acid is only partly split, so less H+ is free
    • The weak acid holds more total H+ in the flask
    • The weak acid reacts with water to remove H+

  5. A weak acid has Ka = 1.0 x 10-4. What is its pKa?

    Answer: ______________

  6. Which two guesses sit inside the 0.10 M ethanoic acid pH working?

    • Water gives almost no H+, and 0.10 stays near 0.10
    • Ka changes with concentration, and water splits fully
    • All the acid splits, and Kw can be ignored

  7. Two acids have Ka values 100 times apart. How far apart are their pKa values?

    • 100 units apart
    • 2 units apart
    • 10 units apart

  8. A student says a higher Ka means a weaker acid. What is the error?

    • Ka only applies to strong acids, not weak ones
    • Higher Ka means more splitting, so a stronger acid
    • Ka has no link to acid strength at all
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Answer key

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Weak acids, Ka and pKa W1-mt_q7fYb7qzMH-s1

  1. X, since lower pKa wins · Lower pKa means larger Ka and a stronger acid, so X wins.
  2. [H+][A-]/[HA] · Products of the split go on top: [H+][A-] over the unsplit [HA].
  3. True · Both flasks hold the same total neutralisable amount because each acid gives one proton per molecule. The sentence is true.
  4. The weak acid is only partly split, so less H+ is free · Only a small share of the weak acid has split, so free [H+] is lower and pH higher.
  5. 4 · pKa = -log(1.0 x 10-4) = 4.
  6. Water gives almost no H+, and 0.10 stays near 0.10 · Both subtractions change nothing: water H+ is tiny and barely any acid splits.
  7. 2 units apart · Each pKa unit is a factor of ten, so a factor of 100 is two units.
  8. Higher Ka means more splitting, so a stronger acid · Ka grows with the split, so a bigger Ka means a stronger weak acid.
Worksheet · LightMySky