Acid-Base Titration Method for AQA A-Level Chemistry

Introduction

• Purpose: Walkthrough of answering an exam question on acid-base titration.
• Audience: Students preparing for the second part of the first required practical in AQA A-Level Chemistry.
• Tip: Six-mark question should take about six minutes to answer.

Initial Preparations

• Equipment: Important to name all equipment and justify steps.
• Buret Preparation:
  • Free of contamination.
  • Rinse with sodium hydroxide (known concentration).
  • Cannot rinse with water as it dilutes sodium hydroxide.

Setting Up the Buret

• Overfill buret past zero.
• Let some solution into a waste beaker to fill the jet space.
  • Prevents inaccurate readings by ensuring the first added volume reaches the conical flask.

Measuring Acid

• Use a volumetric pipette and pipette filler.
  • Transfer 25 cm³ of hydrochloric acid to a conical flask.
  • Avoid using a dropping pipette.
• Conical Flask: Prevents splashing during swirling.

Indicator

• Add a few drops of a suitable indicator (e.g., phenolphthalein).
  • Important for detecting the end point.
  • Avoid excess to prevent reaction with reactants.

Titration Process

• Setup Complete:
  • Known concentration in buret.
  • Unknown concentration in conical flask with indicator.
• Procedure:
  • Start adding alkali from buret while swirling to homogenize solution.
  • Rinse buret end and conical flask sides with deionized water.
• End Point:
  • Look for first permanent color change.
  • Record the volume of alkali added at the end point.

Repeated Trials

• Conduct multiple titrations.
  • First titration is rough and discarded.
  • Continue until results are concordant (within 0.1 cm³ of each other).

Calculations

• Calculate a mean titre from concordant results.
• Use formula: Moles = Concentration × Volume.
  • Determine moles of sodium hydroxide added.
• Reaction Ratio:
  • Sodium hydroxide and hydrochloric acid react in a 1:1 ratio.
• Calculate concentration of hydrochloric acid:
  • Moles of HCl = Moles of NaOH.
  • Concentration = Moles/Volume.

Conclusion

• Recap: Confidence in writing the method for AQA chemistry practical.
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