Practicals

Question 1

How do you investigate paper chromatography using inks/food colourings?

Answer 1

1) Draw pencil line near bottom of chromatography paper
2) Pot spots of each colouring on the line using capillary tube, then the unknown one
3) Label each spot
4) Pour water into the beaker so it sits below the pencil line
5) All solvent to travel up the paper
6) Draw another pencil line just where the solvent has reached, this is called the solvent front line
7) Measure distance between 2 pencil lines, this is the distance travelled by the water solvent
8) For each colouring, measure distance from start line to middle of the spot

Question 2

How do you calculate the Rf values in a chromatography practical?

Answer 2

Divide the distance travelled by the component by the distance travelled by the solvent.

Question 3

How do you determine the formula of a metal oxide by combustion? (Magnesium oxide)

Answer 3

1) Measure the mass of the crucible with the lid
2) Add a sample of magnesium into the crucible and measure the mass with the lid
3) Calculate mass of metal by subtracting mass of crucible
4) Strongly heat crucible over a bunsen burner for several minutes
5) Lift lid frequently to allow air in for Mg to fully oxidise without letting magnesium oxide smoke escape
6) Continue heating until mass remains constant, indicating reaction is complete
7) Measure mass of crucible and its content (calculate mass of metal oxide by subtracting mass of empty crucible
8) Work out empirical formula

Question 4

How do you work out empirical formula of combustion of magnesium oxide?

Answer 4

1) Calculate mass of metal (subtract mass of empty crucible)
2) Calculate mass of oxygen (subtract mass of metal from metal oxide)
3) Work out moles of each (moles = mass/RFM)
4) Simplify the ratio into a whole number ratio

Question 5

How do you determine the formula of a metal oxide by reduction? (Copper (II) oxide)

Answer 5

1) Measure mass of empty boiling tube
2) Place metal oxide into a horizontal boiling tube and measure the mass again
3) Support tube in a horizontal position held by clamp
4) A steady stream of natural gas (methane) is passed over the copper (II) oxide and the excess gas is burned off
5) The copper (II) oxide is heated strongly using a bunsen burner
6) Heat until metal oxide completely changes colour, indicating all oxygen is removed
7) Measure mass of the tube remaining metal powder and subtract mass of tube
8) Work out empirical formula like in combustion method

Question 6

How can you determine the approximate percentage by volume of oxygen in air using a metal?

Answer 6

1) Measure the volume between final mark on the scale and the tap (stopcock) on the burette
2) Fill the burette with water up the lowest mark, 50ml, then let it drain into the small measuring cylinder
3) Measure volume of water
4) Add a little water to moisten the inside of the burette
5) Make sure the tap is closed and sprinkle some iron fillings or push a piece of iron wool into the bottom of the burette
6) Invert the burette into a trough of water and clamp the burette vertically
7) Note and record the position of the water level
8) After 3-4 days note the new position of the water level
9) Use the equation % of O2 in air = change of height of water in burette/ initial volume of air in burette x 100

Question 7

How can you determine the approximate percentage by volume of oxygen in air using a non-metal?

Answer 7

1) Place phosphorus in an evaporating dish and float the dish in a trough of water
2) Ignite the phosphorus and quickly place a bell jar over the water trough, covering the dish. Make sure there is space under the bell jar for water in the trough to move into the jar
3) Note starting height of water level in bell jar
4) Leave apparatus until phosphorus is extinguished
5) Measure final water level in bell jar, the decrease in the volume of air is the volume of oxygen originally in jar
6) Calculate percentage of oxygen in air using change in water level divided by the original volume of air in the bell jar and multiplied by 100

Question 8

How can you investigate reactions between dilute hydrochloric and sulfuric acids and metals (magnesium, iron and zinc)?

Answer 8

1) Add 5cm3 dilute hydrochloric acid to 3 different test tubes
2) Add 1cm length magnesium ribbon to one, observe and note down what you see
3) Test gas given off with a lighted split
4) Add a few pieces of iron fillings to second tube, then to the third some zinc turnings
5) Observe, test gases and note observations
6) Repeat experiment with dilute sulfuric acid

Question 9

What material should the wire be in a flame test?

Answer 9

Platinum or nichrome

Question 10

Describe how to carry out a flame test:

Answer 10

1) Dip a platinum or nichrome wire into concentrated hydrochloric acid to remove any impurities
2) Dip the wire into some of the salt and cover it
3) Hold the wire and salt in a roaring bunsen burner flame
4) Observe the colour

Question 11

Describe simple calorimetry experiments for displacement reactions:

Answer 11

1) Measure and transfer 50cm3 of copper (II) sulfate into a polystyrene cup
2) Measure initial temperature of copper (II) sulfate solution
3) Add magnesium and measure and record maximum temperature
4) Calculate temperature rise

Question 12

Describe simple calorimetry experiments for dissolving reactions:

Answer 12

1) Place a polystyrene cup in a 250cm3 glass beaker
2) Transfer 100cm3 of water into the polystyrene cup using a measuring cylinder
3) Record initial temperature of water
4) Weigh 5.2g of ammonium chloride using a weighing boat on a balance
5) Add ammonium chloride to water and stir the solution vigorously until all the ammonium chloride has dissolved
6) Record the minimum temperature

Question 13

Describe simple calorimetry experiments for neutralisation reactions:

Answer 13

1) Place a polystyrene cup in a 250cm3 glass beaker
2) Transfer 25cm3 of 2.00mol/dm3 potassium hydroxide into the polystyrene cup using a measuring cylinder
3) Record the initial temperature
4) Fill a burette with 50cm3 of dilute hydrochloric acid
5) Use the burette to add 5cm3 of dilute HCl to the KOH
6) Stir vigorously and record the maximum temperature reached
7) Continue adding further portions of 5cm3 HCl and recording maximum temperature until all 50cm3 have been added

Question 14

Describe simple calorimetry experiments for combustion reactions:

Answer 14

1) Measure initial mass of spirit burner containing fuel (eg ethanol)
2) Transfer 100cm3 of water into a copper container and record initial temperature
3) Place burner underneath copper container and light
4) Stir water consistently with thermometer
5) Extinguish flame and measure maximum temperature
6) Reweigh spirit burner

Question 15

Describe reversible reactions such as the dehydration of hydrated copper (II) sulfate:

Answer 15

1) Heat hydrated copper (II) sulfate to get anhydrous copper sulfate and water (colour change from blue to white)
2) Add water to anhydrous copper sulfate to get hydrated copper (II) sulfate (colour change from white to blue)

Question 16

Describe reversible reactions such as heating ammonium chloride:

Answer 16

On heating, white solid ammonium chloride decomposes forming ammonia and hydrogen chloride gas.

On cooling, ammonia and hydrogen chloride react to form a white solid of ammonium chloride.

Question 17

Describe an experiment to prepare a pure, dry sample of soluble salt, starting from an insoluble reactants (making copper (II) sulfate crystals)

Answer 17

1) Measure 50cm3 dilute sulfuric acid into a beaker and heat it on a tripod and gauze using a bunsen burner
2) Add a spatula of black copper (II) oxide and continue heating. Keep adding it until there is some left in the beaker. Stir well to make sure that no more will react (it’s in excess). We know the acid has been neutralised
3) Filter off excess copper (II) oxide and transfer the filtrate (solution), which is blue, to an evaporating basin. The solution we now have is copper (II) sulfate
4) Heat this new solution over a bunsen burner to boil off some of the water and concentrate the solution
5) Keep heating until a saturated solution is formed. We can test this by dipping a glass rod into the solution. If crystals form on the glass rod when we remove it we know that the solution is very close to saturated and crystals will also begin to form in the solution.
6) Stop heating and allow mixture to cool slowly at room temperature so that larger crystals can form
7) Remove the blue crystals from the reaction mixture by filtration
8) Dry crystals with filter paper

Question 18

What is the purpose of cotton wool in a marble chip and hydrochloric acid reaction?

Answer 18

To allow carbon dioxide to escape, but to stop any acid from spilling out.