Preparing Salts

Question 1

Making a soluble salt from an acid with an insoluble base, carbonate or metal

Answer 1

• half fill a beaker with the acid solution
• add the solid base, carbonate or metal a spatula at a time until at excess
• filter to remove excess solid
• evaporate off most of the water and leave to allow salt to crystallise

Question 2

making a soluble salt from an acid + alkali both in solution

Answer 2

• carry out an acid alkali titration using an indicator to see the end point
• once you know the average titre, repeat the titration without an indicator, adding that precise volume from the burette
• evaporate off the water to get salt crystals
• dry the crystals

Question 3

making an insoluble salt from solutions of 2 soluble salts by precipitation

Answer 3

• mix together solutions of 2 suitable soluble salts
• the insoluble salt will appear as a solid precipitate. filter the mixture. the salt is the solid on the filter paper
• wash it by rinsing with distilled water
• scrape onto a watch glass of fresh filter paper to dry

Question 4

preparation of Copper (II) Sulphate

Answer 4

• measure 20 cm cubed of dilute sulphuric acid using a measuring cylinder and pour it into a small conical flask
• warm the acid using a Bunsen burner, use a thermometer to measure the temperature
• add a little copper oxide powder to the acid and stir
• if all the copper oxide reacts and disappears, add a little more. stop when in excess
• filter the mixture and transfer the filtrate to an evaporating basin
• heat the evaporating basin, over a water bath, until half the water has been evaporated
• pour the solution into a crystallising dish and allow all the water to evaporate.

Question 5

during preparation of copper (II) sulphate, why was it necessary to warm the sulphuric acid?

Answer 5

• to increase the reaction time

Question 6

during preparation of copper (II) sulphate, why was it necessary to add copper oxide in excess?

Answer 6

• to ensure all the acid has reacted fully with the solid

Question 7

how can you tell that the copper oxide was in excess?

Answer 7

• because the powder was beginning to gather at the bottom of the beaker and wouldn’t dissolve away

Question 8

what was the filtrate and residue in the preparation of copper (II) sulphate?

Answer 8

filtrate: CuSO4 solution
residue: excess copper oxide powder

Question 9

why should you not evaporate all the water from the copper sulphate solution?

Answer 9

• to prevent driving off the water of evaporation

Question 10

write a balanced equation for the reaction occurring during preparation of copper (II) sulphate

Answer 10

H2SO4 + CuO => CuSO4 + H2O