Buffers and Titration Curves · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Buffers and titration curves

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

  1. In a buffer with equal amounts of acid and salt, what is the pH?

    • Always exactly 7
    • Equal to Kw
    • Equal to pKa

  2. What deals with added acid in an acetic acid and sodium acetate buffer?

    • The conjugate base, turning into HA
    • The weak acid, turning into A-
    • The water, turning into steam

  3. An indicator works when its range sits fully inside the steep part of the curve.

    Circle one:   True   False

  4. Where does the steep run sit when a weak acid meets a strong base?

    • Below pH 7
    • Exactly at pH 7
    • Above pH 7

  5. How does blood shrug off the acid from hard exercise?

    • The kidneys filter out all the carbonic acid at once
    • Extra H+ becomes CO2, which is breathed out
    • The blood dilutes itself with extra water

  6. Which indicator suits ammonia titrated with hydrochloric acid, and why?

    • Methyl orange, since its range sits on the steep fall
    • Phenolphthalein, since its range sits on the steep fall
    • No indicator, since the colour never changes

  7. A student adds a lot of strong acid to a buffer and expects the pH to hold. What is the flaw?

    • Buffers resist any amount of acid with no limit
    • Buffers only resist small additions, since the stored base runs out
    • Buffers only ever handle added alkali, never added acid

  8. A buffer has twice as much salt as acid. A student claims its pH equals pKa. What is wrong?

    • The pH sits above pKa, since log(2) is positive
    • The pH sits below pKa, since log(2) is negative
    • The buffer stops working when the ratio is not 1
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Buffers and titration curves W1-mt_vevyEjNijM-s1

  1. Equal to pKa · With log(1) = 0, the ratio term vanishes and pH = pKa.
  2. The conjugate base, turning into HA · Added H+ is eaten by the conjugate base forming HA, so pH barely moves.
  3. True · Then one drop near equivalence flips the colour sharply. The sentence is true.
  4. Above pH 7 · The salt left at equivalence is slightly basic, so the run sits above 7.
  5. Extra H+ becomes CO2, which is breathed out · The carbonic acid and hydrogencarbonate buffer turns spare H+ into CO2 for the lungs.
  6. Methyl orange, since its range sits on the steep fall · The steep fall sits near pH 3 to 4, inside the methyl orange range.
  7. Buffers only resist small additions, since the stored base runs out · The conjugate base is eaten as it works, so a large dose exhausts it and the pH falls.
  8. The pH sits above pKa, since log(2) is positive · pH = pKa + log(2), and log(2) is positive, so the pH lands above pKa.
Worksheet · LightMySky