Acid-base titrationsEdexcel International A Level Chemistry: Subtopic test

1. 1
   In a titration, 0.100 mol dm⁻³ sodium hydroxide is run from a burette into 25.0 cm³ of hydrochloric acid of unknown concentration in a conical flask. Methyl orange and phenolphthalein are both available as indicators.

   (a)

   The methyl orange indicator is placed in the acid in the flask. What colour change is seen as the sodium hydroxide is added to the end point?

   [1 mark]

   • AYellow to red
   • BRed to yellow, passing through orange at the end point
   • CColourless to pink
   • DPink to colourless

   (b)

   Which statement about the choice of indicator is correct for this titration?

   [1 mark]

   • AOnly methyl orange is suitable, because the flask contains an acid
   • BOnly phenolphthalein is suitable, because the burette contains an alkali
   • CNeither is suitable, because the end point is exactly pH 7
   • DEither is suitable, because the pH changes sharply through both colour-change ranges at the end point

   (c)

   The end point is reached when 21.40 cm³ of sodium hydroxide has been added. Calculate the concentration of the hydrochloric acid in mol dm⁻³.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student prepares a standard solution of ethanedioic acid dihydrate, H₂C₂O₄·2H₂O (M = 126.1 g mol⁻¹). She weighs 1.58 g of the solid in a beaker, dissolves it in deionised water and makes the solution up to 250 cm³ in a volumetric flask.

   (a)

   What is the concentration of the standard solution in mol dm⁻³?

   [1 mark]

   • A0.0501
   • B0.0125
   • C0.00313
   • D0.200

   (b)

   Which procedure should the student use to transfer the solid into the volumetric flask?

   [1 mark]

   • ATip the solid through a dry funnel and fill to the mark with tap water
   • BDissolve the solid in 250 cm³ of water in a beaker and pour it into the flask
   • CDissolve the solid in a little water in the beaker, transfer it, and rinse the beaker and funnel into the flask
   • DWeigh the solid directly into the flask and heat the flask to dissolve it

   (c)

   Explain why the student adds the last of the water drop by drop until the bottom of the meniscus is on the graduation mark, and then inverts the flask several times.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The student uses the standard ethanedioic acid solution, of concentration 0.0501 mol dm⁻³, to find the concentration of a solution of sodium hydroxide. She fills a burette with the acid and titrates 25.0 cm³ portions of the sodium hydroxide solution, using phenolphthalein. The equation is H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O. The mean of her concordant titres is 22.60 cm³.

   (a)

   Describe how the student should carry out one titration so that the end point is found accurately.

   [3 marks]

   (b)

   Calculate the concentration of the sodium hydroxide solution in mol dm⁻³ and in g dm⁻³. (M of NaOH = 40.0 g mol⁻¹)

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student titrates 25.0 cm³ portions of hydrochloric acid, measured with a pipette, against 0.100 mol dm⁻³ sodium hydroxide from a burette. The mean titre is 24.20 cm³. The uncertainty of each burette reading is ±0.05 cm³ and the uncertainty of the 25.0 cm³ pipette is ±0.06 cm³. The uncertainty in the concentration of the sodium hydroxide can be ignored.

   (a)

   Calculate the concentration of the hydrochloric acid in mol dm⁻³. Calculate the percentage uncertainty in the titre, and in the volume of acid measured by the pipette, and use these to give the concentration with its absolute uncertainty.

   [6 marks]

   (b)

   Suggest three changes to the procedure that would improve the accuracy or reliability of the result, and explain the effect of each change.

   [6 marks]

   Total for question 4: 12 marks

End of questions