Experiments

Question 1

Outline the method for determining % by mass of CuCO3.Cu(OH)2

Answer 1

1) Weigh out 1.5g of CuCO3.Cu(OH)2 on mass balance.
2) Add to conical flask.
3) Clamp gas syringe, insert tubing into conical flask. Record initial volume.
4) Measure out 50cm3 of H2SO4 in measuring cyclinder.
5) Add H2SO4 to conical flask and quickly stopper flask with gas syringe.
6) When plunger stops moving, measure final volume in gas syringe.

Question 2

What are the practical considerations in the experiment for determining % by mass of CuCO3.Cu(OH)2?

Answer 2

• Escaped gas - some CO2 will inevitably escape in the period between adding the sulfuric acid and stoppering the flask. We can minimise this loss of gas by having the gas syringe ready to attach immediately after adding the sulfuric acid.
• If gas is collected in measuring cyclinder over water - this method has an additional source of gas loss: some CO2 will dissolve in the water rather than filling the measuring cylinder.
• Gas syringe must be horizontal so that the syringe doesn’t have to work against gravity to expand.

Question 3

Outline the steps on how you find the % by mass of CuCO3 in CuCO3.Cu(OH)2

Answer 3

1) Calculate the moles of CO2 collected.
2) Calculate the moles of CuCO3 that reacted (should be 1:1 ratio)
3) Find mass of CuCO3 in sample using n x Mr.
4) Compare to mass of CuCO3.Cu(OH)2

Question 4

Suggest a method that would allow you to check that the reaction is complete when reacting something with oxygen in a crucible

Answer 4

After weighing the product in the crucible, heat the crucible again for a couple of minutes and re-weigh it. Repeat this process until the mass readings stop changing, which indicates that the reaction is complete. This method is known as heating to constant mass.

Question 5

What is recrystallisation used for?

Answer 5

Purifying a solid

Question 6

Outline the steps of recrystallisation

Answer 6

1) Dissolve the solid in a minimum quantity of hot solvent.
2) Remove any insoluble impurities using gravity filtration.
3) Cool the mixture.
4) Filter the crystals under reduced pressure, washing with cold solvent.
5) Leave the crystals to dry.

Question 7

What are the practical considerations in recrystalisation?

Answer 7

• We need solvent our wanted product will dissolve in at high temperatures, but form crystals in at low temperatures.
• We dissolve impure solid in minimum quantity of hot solvent, so it can easily cool.
• We need to keep solution hot when filtering insoluble impurities, or some of the wanted product could recrystalise on filter paper and would be thrown away with the insoluble impurities.
• We leave our solution to cool for maximum amount of time, to let as many pure crystals form as possible.
• Crystals washed with cold solvent during filtration to remove any remaining soluble impurities.