1. 1
   A student carries out a titration to find the volume of dilute hydrochloric acid needed to exactly neutralise a fixed volume of sodium hydroxide solution, using a suitable indicator to identify the end-point.

   (a)

   Which piece of apparatus should the student use to measure out the fixed volume of sodium hydroxide solution accurately into a conical flask?

   [1 mark]

   • AVolumetric pipette
   • BBurette
   • CMeasuring cylinder
   • DGas syringe

   (b)

   The student adds a few drops of indicator to the sodium hydroxide solution before starting the titration. Which indicator would be suitable for identifying the end-point of this reaction?

   [1 mark]

   • AUniversal indicator only, since it gives a numerical pH value
   • BMethyl orange
   • CLimewater
   • DAqueous silver nitrate

   (c)

   Describe how the student would identify the end-point of the titration and explain what has happened chemically at this point.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A quality control chemist at a fertiliser plant uses a titration to determine the concentration of an ammonium sulfate solution, by reacting a sample of it with a sodium hydroxide solution of known concentration.

   (a)

   Before starting, the chemist rinses the burette with a small volume of the solution it will contain. Why is this step necessary?

   [1 mark]

   • ATo colour the solution so the end-point is easier to see
   • BTo increase the concentration of the solution being tested
   • CTo neutralise any acid left in the burette from a previous experiment
   • DTo remove any water or previous solution that could dilute the solution added, affecting the results

   (b)

   The chemist carries out the titration three times and obtains volumes of 24.60 cm3, 24.55 cm3 and 27.80 cm3 of sodium hydroxide solution used. Which value(s) should the chemist use to calculate an accurate average (concordant) titre?

   [1 mark]

   • A27.80 cm3 only, since it was the last reading taken
   • BAll three values, since more data always gives a more accurate average
   • C24.60 cm3 and 24.55 cm3 only, since 27.80 cm3 is not concordant with these
   • DThe largest and smallest values only

   (c)

   Suggest one possible reason why the third titre (27.80 cm3) might have been so much higher than the first two, in terms of a procedural error during the titration.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A pharmaceutical laboratory uses titration to check the purity of a batch of citric acid used in a medicine, by titrating a sample against a standard sodium hydroxide solution of known concentration. The laboratory technician repeats the titration until three concordant results are obtained.

   (a)

   Describe the steps needed to correctly set up and use a burette to carry out this titration accurately.

   [3 marks]

   (b)

   Explain why the technician repeats the titration until three concordant results are obtained, rather than relying on a single titration, and describe how the technician should then calculate the value to be used in further calculations.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school is training students to carry out an acid-alkali titration between dilute sulfuric acid and potassium hydroxide solution for the first time, using phenolphthalein as the indicator.

   (a)

   Describe, in a logical sequence, the full procedure the students should follow from setting up the apparatus to obtaining a set of concordant titres, explaining the purpose of each key step.

   [6 marks]

   (b)

   One group of students obtains titres of 22.40 cm3, 22.45 cm3 and 26.10 cm3 across their three attempts. Explain how the students should treat these results, and discuss the sources of error that might explain both the anomalous result and any small variation between the two closely agreeing results.

   [6 marks]

   Total for question 4: 12 marks

End of questions