Solution concentrations and titration calculationsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A school technician prepares sodium hydroxide and hydrochloric acid solutions of known concentration for a titration practical. Use relative atomic masses H = 1.0, O = 16.0, Na = 23.0 and Cl = 35.5.

   (a)

   2.00 g of sodium hydroxide is dissolved in water and made up to 250.0 cm³. What is the concentration of the solution in mol dm⁻³?

   [1 mark]

   • A0.0125 mol dm⁻³
   • B0.0500 mol dm⁻³
   • C0.200 mol dm⁻³
   • D8.00 mol dm⁻³

   (b)

   A solution of hydrochloric acid has a concentration of 0.200 mol dm⁻³. What is its concentration in g dm⁻³?

   [1 mark]

   • A7.30 g dm⁻³
   • B0.00548 g dm⁻³
   • C182.5 g dm⁻³
   • D36.5 g dm⁻³

   (c)

   Calculate the mass of sodium hydroxide needed to make 500 cm³ of a solution of concentration 0.150 mol dm⁻³.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student titrates 25.0 cm³ portions of sodium hydroxide solution of unknown concentration with 0.100 mol dm⁻³ hydrochloric acid from a burette, repeating the titration until concordant results (within 0.10 cm³ of each other) are obtained. The equation is NaOH + HCl → NaCl + H₂O.

   (a)

   Phenolphthalein is added to the sodium hydroxide in the conical flask. Which colour change shows the end-point as the acid is run in?

   [1 mark]

   • AColourless to pink
   • BYellow to red
   • CRed to yellow
   • DPink to colourless

   (b)

   The titres obtained were 23.10 (rough), 21.80, 21.65, 21.55 and 21.60 cm³. What is the mean titre that should be used in the calculation?

   [1 mark]

   • A21.65 cm³
   • B21.60 cm³
   • C21.94 cm³
   • D23.10 cm³

   (c)

   The mean titre is 21.60 cm³. Calculate the concentration of the sodium hydroxide solution in mol dm⁻³.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares a standard solution of ethanedioic acid by dissolving 1.575 g of ethanedioic acid dihydrate, H₂C₂O₄·2H₂O (M = 126.0 g mol⁻¹), in water and making the solution up to 250.0 cm³ in a volumetric flask. The student then titrates 25.0 cm³ portions of this solution against sodium hydroxide solution of unknown concentration, using phenolphthalein, and obtains a mean titre of 22.40 cm³. The equation is H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O.

   (a)

   Calculate the concentration of the standard solution of ethanedioic acid in (i) mol dm⁻³ and (ii) g dm⁻³ of the dihydrate.

   [3 marks]

   (b)

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

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In two core practicals, a class first prepares a standard solution of a solid acid and uses it to find the concentration of a sodium hydroxide solution. The class then uses this sodium hydroxide solution, now of known concentration, to find the concentration of a solution of hydrochloric acid, using phenolphthalein indicator.

   (a)

   Describe how a standard solution of a solid acid should be prepared, explaining why each step is carried out.

   [6 marks]

   (b)

   Describe how the hydrochloric acid solution should be titrated with the sodium hydroxide solution so that the result is accurate and reliable. Include the indicator and its colour change.

   [6 marks]

   Total for question 4: 12 marks

End of questions