Redox Titrations with Manganate(VII) and Thiosulfate · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Redox titrations that show their own end

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

  1. What does the end point of a manganate titration look like?

    • The pink colour vanishes for good
    • A precipitate falls to the bottom
    • A first faint pink that stays

  2. Why is no separate indicator added in a manganate(VII) titration?

    • Manganate acts as its own indicator
    • Indicators do not work in acid solution
    • The end point needs no colour change

  3. In iodine titrations, starch should be added only near the end.

    Circle one:   True   False

  4. What must you build from the half equations before any titration maths?

    • The full ionic equation for the reaction
    • The pH curve for the mixture
    • The rate equation for the reaction

  5. A titration uses 25.0 cm3 of 0.020 M manganate. How many moles of manganate is that?

    Answer: ______________

  6. Why is starch held back until near the end of an iodine titration?

    • Starch reacts with thiosulfate and ruins it
    • Early starch locks onto iodine and blurs the end point
    • Starch stops the iodine from forming at all

  7. 2.5 x 10-3 mol of iron is found in a tablet. With iron at 55.8 g per mole, what is the iron mass in grams?

    Answer: ______________

  8. A student multiplies titre moles by 1:1 for manganate with iron(II). What is the error?

    • The ratio is 1 to 5, since manganate takes 5 electrons
    • The ratio is 5 to 1, since iron gives 5 electrons
    • Ratios never matter in redox titrations
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Answer key

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

Redox titrations that show their own end W1-mt_tKZTSfo0Ra-s1

  1. A first faint pink that stays · The lasting pink is the first extra manganate with nothing left to react with.
  2. Manganate acts as its own indicator · Excess manganate tints the flask pink, so it shows its own end point.
  3. True · Early starch locks onto iodine and drags the finish. The sentence is true.
  4. The full ionic equation for the reaction · The full ionic equation fixes the mole ratio that every later step needs.
  5. 0.0005 · 0.0250 dm3 times 0.020 mol/dm3 = 5.0 x 10-4 mol.
  6. Early starch locks onto iodine and blurs the end point · Starch grips plentiful iodine too tightly to let go cleanly, so the finish drags.
  7. 0.1395 · 2.5 x 10-3 times 55.8 = 0.1395 g.
  8. The ratio is 1 to 5, since manganate takes 5 electrons · Five iron(II) particles feed one manganate, so the titre moles must be timesed by 5.
Worksheet · LightMySky