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CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations – Study Notes

CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

Core

  • Describe an acid–base titration to include the use of a:
    (a) burette
    (b) volumetric pipette
    (c) suitable indicator
  • Describe how to identify the end-point of a titration using an indicator

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Acid–Base Titration

An acid–base titration is an experimental technique used to determine the concentration of an acid or alkali by reacting it with a base or acid of known concentration, using an indicator to show the endpoint.

(a) Burette

  • A burette is a long, graduated glass tube with a tap at the bottom.
  • It is used to deliver variable, precise volumes of the solution of known concentration (the titrant).
  • In a typical titration, the burette is filled with the acid or the alkali of known concentration.

(b) Volumetric Pipette

  • A volumetric pipette is used to measure and transfer a fixed, accurate volume of the solution of unknown concentration into a conical flask (e.g. 25.0 cm³).
  • A pipette filler is used to safely draw up the liquid.

(c) Suitable Indicator

  • An indicator shows when neutralisation has occurred (the endpoint).
  • Common indicators:
    • Phenolphthalein: Colourless in acid, pink in alkali.
    • Methyl orange: Red in acid, yellow in alkali, orange at endpoint.
  • Universal indicator is not used because it has too many colour changes, making the endpoint unclear.

Procedure:

  1. Use a volumetric pipette to measure 25.0 cm³ of alkali and transfer it to a conical flask.
  2. Add a few drops of a suitable indicator (e.g. phenolphthalein or methyl orange).
  3. Fill the burette with acid of known concentration, making sure the meniscus is read at eye level.
  4. Slowly add the acid from the burette to the alkali, swirling the flask continuously.
  5. Stop when the indicator changes colour sharply (endpoint).
  6. Repeat the titration several times until concordant results (within 0.1 cm³) are obtained.

Key Idea: The volume of acid (from the burette) required to neutralise the known volume of alkali (from the pipette) is used to calculate the concentration of the unknown solution.

Example :

25.0 cm³ of sodium hydroxide solution was titrated with hydrochloric acid of concentration 0.100 mol/dm³. The mean titre was 24.8 cm³. Calculate the concentration of the sodium hydroxide solution.

▶️ Answer/Explanation

Step 1: Balanced equation: \(\mathrm{HCl + NaOH \;\;\rightarrow\;\; NaCl + H_2O}\).

Step 2: Moles of \(\mathrm{HCl} = \dfrac{0.100 \times 24.8}{1000} = 0.00248 \, \text{mol}\).

Step 3: Ratio HCl : NaOH = 1 : 1 → moles of NaOH = 0.00248 mol.

Step 4: Concentration of NaOH = \(\dfrac{0.00248}{25.0/1000} = 0.0992 \, \text{mol/dm}^3\).

Final Answer: The concentration of NaOH = \(0.099 \, \text{mol/dm}^3\) (3 s.f.).

Identifying the End-Point of a Titration

 The end-point of a titration is the stage at which the reaction between the acid and the base is just complete. It is detected using a suitable indicator that changes colour sharply at neutralisation.

How to Identify the End-Point:

  • Add a few drops of a suitable indicator to the solution in the conical flask (e.g. alkali with indicator, acid in burette).
  • Swirl the flask while slowly adding the solution from the burette.
  • As neutralisation is reached, the indicator will change colour sharply and permanently.
  • The exact moment this colour change occurs is the end-point of the titration.

Common Indicators:

  • Phenolphthalein: Colourless in acid, pink in alkali → endpoint when the pink colour just disappears.
  • Methyl orange: Red in acid, orange at endpoint, yellow in alkali.

Key Idea: The endpoint is shown by a distinct colour change in the indicator, not a gradual one.

Example :

In a titration between hydrochloric acid and sodium hydroxide, phenolphthalein was used as the indicator. Describe how the endpoint is identified.

▶️ Answer/Explanation

Step 1: At the start, the sodium hydroxide solution is alkaline, so phenolphthalein is pink.

Step 2: As hydrochloric acid is added from the burette, the alkali is gradually neutralised.

Step 3: At the endpoint, the pink colour just disappears, leaving the solution colourless.

Final Answer: The endpoint is identified when phenolphthalein changes from pink to colourless.

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