2. Preparing Salts

Question 1

Why is the insoluble reactant added in excess to the acid during the preperation of soluble salts?

Answer 1

To ensure that all of the acid has reacted since any unreacted acid would become dangerously concentrated during evaporation and crystallisation.

Question 2

How do you prepare a soluble salt from an insoluble base and acid?

(Using the example of making copper sulfate)

Answer 2

• React a fixed volume of acid (sulfuric acid) with an excess of a solid insoluble base (copper oxide) and heat the solution, stirring with a glass rod until the reaction finishes
• Use filtration with a funnel and filter paper to ensure that only the soluble salt and water remains (the excess insoluble base will be filtered out)
• Crystallise this solution by heating it to evaporate some (around half) of the solvent in the solution so it becomes saturated and then removing the heat and allowing crystals to form as it cools for a few hours
• Filter out the remain solution by decanting it, then collect the crystals and dry them on/with a paper tower or evaporating dish in a warm place (such as a warm oven)

Question 3

How do you prepare a soluble salt from an acid and alkali?

Answer 3

• Measure alkali into a conical flask with indicator.
• Add acid from burette slowly until indicator changes.
• Record initial and final burette readings → calculate acid volume used.
• Repeat reaction with same volumes but no indicator.
• Heat to evaporate water → form saturated solution.
• Leave to cool and crystalise
• Filter off the cyrstals/ pour off any excess liquid
• Dry crystals using a warm oven.

Question 4

What is a percipitate?

Answer 4

A solid formed when two solutions are mixed.

Question 5

How do you prepare an insoluble salt?

(Using the example of making lead(II) sulfate)

Answer 5

• Mix two solutions of soluble salts together (Lead nitrate and another soluble sulfate salt) and stir
• Filter it to remove the precipitate (insoluble salt) from the rest of the solution, wash it with distilled water and then dry it using a warm oven.
• You should be left with a pure, dry sample of an insoluble salt, in this case lead(II) sulfate

• The precipitate forms because lead sulfate is insoluble, everything else is in the solution because it is soluble
• If the two soluble salts are in solid form, dissolve them both in water first then continue

Question 6

Filtration Diagram

Answer 6

Question 7

Prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide

Answer 7

• Add 50 cm³ dilute sulfuric acid to a beaker and warm gently with a Bunsen burner.
• Add copper(II) oxide slowly, stirring until no more dissolves or no more bubbling takes place (excess solid remains).
• Filter to remove the excess base, collecting the filtrate in an evaporating basin.
• Heat the filtrate gently using a Bunsen burner until it becomes saturated.
• Test saturation by dipping in a cold glass rod – crystals form if saturated.
• Leave the solution in a warm place to crystallise.
• Decant remaining liquid and allow crystals to dry.

Question 8

What do Hydrated copper(II) sulfate crystals look like

Answer 8

Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped

Question 9

Prepare Lead(II) Sulfate

Answer 9

• Measure out 25 cm3 of 0.5 mol dm3 lead(II) nitrate solution and add it to a small beaker
• Measure out 25 cm3 of 0.5 mol dm3 of potassium sulfate add it to the beaker and mix together using a stirring rod
• Filter to remove precipitate from mixture
• Wash the lead (II) sulfate with distilled water to remove traces of other solutions
• Leave in an oven to dry

Question 10

Explain why a pipette is used to measure the lithium hydroxide solution but a burette is used to measure the hydrochloric acid.

Answer 10

• Pipette measures one fixed volume (accurately)
• Burette measures variable volumes (accurately)

Question 11

Describe how to use the method of crystallisation to obtain a pure dry sample of the salt.

Answer 11

• Heat the solution to form a saturated solution
• Leave the solution to cool (so crystals form)
• Decant and Place in a warm oven