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Chemistry · Practice book · Senior 6 · Unit 9 of 14

Indicators and titration curves

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 9 Indicators and titration curves

Key unit competence: Evaluate the relationship between titration curves, the type of acid–base titration involved and the choice of indicator.

Key ideas

  • An acid–base indicator is a weak acid (HIn) whose acid form and conjugate base form (In−) have different colours: HIn ⇌ H+ + In−. The colour changes over a range of about pKIn ± 1; the end point is when [HIn] = [In−] (pH = pKIn).
  • Common indicators: methyl orange, pH 3.1–4.4 (red → yellow); methyl red, 4.4–6.2 (red → yellow); bromothymol blue, 6.0–7.6 (yellow → blue); phenolphthalein, 8.3–10.0 (colourless → pink).
  • A titration curve plots pH against volume of titrant. The equivalence point is where moles of acid and base match exactly; the curve is steepest there. Choose an indicator whose range lies in the vertical part of the curve.
  • Strong acid–strong base: start pH 1, vertical part about pH 3–11, equivalence pH 7: any common indicator. Weak acid–strong base: start about pH 3, a buffer region, vertical part about 7–11, equivalence pH about 8.7: phenolphthalein. At half-equivalence pH = pKa.
  • Strong acid–weak base: vertical part about 3–7, equivalence pH about 5.3: methyl orange (or methyl red). Weak acid–weak base: no sharp vertical part, so no indicator is suitable (use a pH meter or conductivity).
  • Sodium carbonate titrated with HCl shows TWO equivalence points: CO32− + H+ → HCO3− at about pH 8.3 (phenolphthalein), then HCO3− + H+ → H2O + CO2 at about pH 3.8 (methyl orange); the second needs twice the volume of the first.

Common misconceptions

  • "The end point and the equivalence point are always the same." No: The end point is when the indicator changes; with a well-chosen indicator it is very close to the equivalence point.
  • "The equivalence point is always at pH 7." No: Only for strong acid–strong base; weak acid–strong base gives pH > 7, strong acid–weak base pH < 7.
  • "Any indicator works for any titration." No: The indicator's range must fall within the steep part of the curve.

Worked examples

Example 1. Sketch (describe) the pH curve for 25.0 cm3 of 0.10 mol dm−3 ethanoic acid titrated with 0.10 mol dm−3 NaOH and mark its key points.

Solution

Start: pH about 2.9 (weak acid).

Buffer region: pH rises slowly; at 12.5 cm3 (half-equivalence) pH = pKa = 4.74.

Equivalence at 25.0 cm3: steep rise from about pH 7 to 11, with the equivalence point near pH 8.7 (ethanoate is basic).

After equivalence: levels off near pH 12.5–13 (excess NaOH). Suitable indicator: phenolphthalein.

Example 2. Explain how an indicator works, using phenolphthalein as an example.

Solution

Phenolphthalein is a weak acid, HIn (colourless); its anion In− is pink: HIn ⇌ H+ + In−.

In acid, high [H+] pushes the equilibrium left: colourless.

When alkali is added, OH− removes H+, the equilibrium shifts right: pink.

The colour changes between pH 8.3 and 10.0, centred on pKIn ≈ 9.3.

Practice questions

  1. Define the end point of a titration.

    Show answer

    The point at which the indicator changes colour.

    Common mistake: Saying it is when the burette is empty.

  2. Why must only a few drops of indicator be used?

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    The indicator is itself a weak acid and would react with the titrant, changing the titre.

    Common mistake: Saying more indicator gives better colour.

  3. Give the pH at equivalence for HCl with ammonia: above, below or equal to 7?

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    Below 7 (about 5.3).

    Common mistake: Saying 7.

  4. What is the pH at the half-equivalence point of a weak acid titration equal to?

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    pKa of the acid.

    Common mistake: Saying 7.

  5. Name a suitable indicator for HCl with NaOH.

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    Methyl orange or phenolphthalein (both fall in the vertical part).

    Common mistake: Saying only universal indicator.

  6. How can the Ka of a weak acid be found from its titration curve?

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    Read the pH at half the equivalence volume; pH = pKa, Ka = 10−pKa.

    Common mistake: Reading the pH at the equivalence point.

  7. Why is universal indicator not used for titrations?

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    It changes colour gradually over a wide range, so there is no sharp end point.

    Common mistake: Saying it is too expensive.

  8. Write the equation for the second stage of the titration of Na2CO3 with HCl.

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    HCO3− + H+ → H2O + CO2

    Common mistake: Writing CO32− + 2H+ → CO2 + H2O as stage 2.

  9. Why does a weak acid–strong base titration curve start at a higher pH than a strong acid one of equal concentration?

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    The weak acid is only partly ionised, so [H+] is lower.

    Common mistake: Saying the weak acid is more dilute.

  10. Name the region of a weak acid titration curve where pH changes slowly.

    Show answer

    The buffer region.

    Common mistake: Calling it the equivalence region.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Methyl orange changes colour in the pH range:a. 3.1–4.4
    2. For a strong acid–strong base titration, the pH at equivalence is:b. pink
    3. Titrating Na2CO3 with HCl gives how many equivalence points?c. 7
    4. Phenolphthalein in alkali is:d. Two
    Show answer

    1 → a (3.1–4.4) · 2 → c (7) · 3 → d (Two) · 4 → b (pink)

    • Red to yellow.
    • Neutral salt.
    • CO32− → HCO3− → H2CO3.
    • In− form.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    equivalence point  ·  pH = 14  ·  pH = pKa  ·  8.3–10.0
    1. The steep part of a titration curve is near the .
    2. Phenolphthalein changes colour in the pH range .
    3. At the half-equivalence point of a weak acid–strong base titration .
    Show answer

    1: equivalence point · 2: 8.3–10.0 · 3: pH = pKa

    • Small volume gives a big pH change.
    • Colourless to pink.
    • [HA] = [A−].

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. An acid–base indicator is usually:a. pKIn
    2. The first equivalence point of Na2CO3 with HCl is detected with:b. a weak acid whose two forms have different colours
    3. The colour change of an indicator happens at about pH =:c. phenolphthalein
    4. Same titration: 30.0 cm3 of NaOH added. Find the pH.d. 11.96
    Show answer

    1 → b (a weak acid whose two forms have different colours) · 2 → c (phenolphthalein) · 3 → a (pKIn) · 4 → d (11.96)

    • HIn ⇌ H+ + In−.
    • About pH 8.3.
    • [HIn] = [In−].
    • Step 1. Excess OH− = 0.00300 − 0.00250 = 5.0 × 10−4 mol in 55.0 cm3.
      Step 2. [OH−] = 9.09 × 10−3; pOH = 2.04.
      Step 3. pH = 11.96.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    changes slowly  ·  red  ·  yellow to blue  ·  yellow
    1. Bromothymol blue changes from .
    2. In the buffer region of a weak acid titrated with NaOH, the pH .
    3. Methyl orange in acid is .
    Show answer

    1: yellow to blue · 2: changes slowly · 3: red

    • Range 6.0–7.6.
    • Mixture of HA and A−.
    • HIn form.

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