1. 1
   A school technician wants to prepare a pure sample of copper(II) sulfate crystals for a lesson. Copper(II) oxide is insoluble in water but reacts with dilute sulfuric acid.

   (a)

   Which method should the technician use to prepare the salt?

   [1 mark]

   • AMix copper(II) oxide with solid sodium chloride and heat strongly
   • BTitrate copper(II) oxide against dilute sulfuric acid using an indicator
   • CAdd excess copper(II) oxide powder to warm dilute sulfuric acid, then filter off the unreacted excess
   • DDissolve copper(II) oxide directly in water with no acid required

   (b)

   Why is it important that the technician adds an excess of copper(II) oxide to the acid, rather than an exact amount?

   [1 mark]

   • ATo speed up the rate of the reaction only
   • BTo make sure some acid remains unreacted in the final solution
   • CExcess is not necessary; an exact amount always gives a purer product
   • DTo make sure all of the acid has reacted, so that the remaining solution contains no unreacted acid

   (c)

   Describe the remaining steps the technician should follow, after filtering off the excess copper(II) oxide, to obtain pure, dry, blue copper(II) sulfate crystals from the filtered solution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student needs to prepare a pure sample of sodium chloride, but both sodium hydroxide and hydrochloric acid are soluble in water, so no solid excess can be filtered off.

   (a)

   Which method should the student use?

   [1 mark]

   • AAdd excess solid hydrochloric acid to the sodium hydroxide solution
   • BAdd excess solid sodium hydroxide to the acid and filter off the excess
   • CTitrate a measured volume of sodium hydroxide against hydrochloric acid using an indicator, then repeat without the indicator using the volumes found
   • DMix the two solutions in any proportion and evaporate immediately

   (b)

   Why must the student repeat the titration without the indicator, using the exact volumes previously found, before evaporating the solution?

   [1 mark]

   • AThere is no need to repeat; the original titration mixture can be evaporated directly
   • BBecause the indicator prevents the reaction from occurring at all
   • CBecause repeating without an indicator changes the volumes of acid and alkali needed
   • DBecause the indicator itself would contaminate the final salt product

   (c)

   State the general solubility rule that confirms sodium chloride will remain dissolved in solution rather than precipitating out during the reaction, and explain how the student would finally obtain solid, pure sodium chloride crystals.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A water-treatment laboratory wants to prepare a pure sample of lead(II) sulfate, an insoluble salt, to study how it forms scale in pipes.

   (a)

   Describe the method of precipitation the laboratory should use, naming two soluble solutions that could be mixed to produce lead(II) sulfate as a precipitate.

   [3 marks]

   (b)

   Using the general solubility rules, explain why lead(II) sulfate forms as a precipitate in this reaction, whereas if lead(II) nitrate had instead been mixed with sodium chloride solution, a different insoluble salt would form, and predict what that salt would be.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two students in a class each heat a hydrated salt until it becomes anhydrous, to determine its water of crystallisation. Student A heats hydrated cobalt(II) chloride, CoCl2·xH2O, and Student B heats hydrated magnesium sulfate, MgSO4·yH2O. (Molar masses: anhydrous CoCl2 = 130 g/mol, anhydrous MgSO4 = 120 g/mol, H2O = 18 g/mol.)

   (a)

   Student A weighs 2.38 g of hydrated cobalt(II) chloride, CoCl2·xH2O, and finds the anhydrous cobalt(II) chloride has a mass of 1.30 g. Calculate the number of moles of water lost per mole of anhydrous cobalt(II) chloride, and use this to state the value of x.

   [6 marks]

   (b)

   Student B weighs 4.92 g of hydrated magnesium sulfate, MgSO4·yH2O, and finds the anhydrous magnesium sulfate remaining has a mass of 2.40 g. Calculate the value of y, showing your working, and compare the amount of water of crystallisation per formula unit in magnesium sulfate with that found for cobalt(II) chloride in the previous experiment.

   [6 marks]

   Total for question 4: 12 marks

End of questions