Gases in the atmosphere (Edexcel IGCSE Chemistry (Modular): Unit 2)

The percentage by volume of oxygen in air can be found by using the rusting of iron. A student sets up this apparatus to measure the volume of oxygen in a sample of air.

An excess of wet iron filings is used. At the start of each experiment, the reading on the syringe is recorded and the apparatus is then left for a week so that the reaction is complete. The reading on the syringe is then recorded again.

The diagram shows the readings in one experiment.

Complete the table to show:

• the syringe reading at the end of this experiment

• the volume of oxygen used in the experiment.

The table shows the results recorded by a different student in her experiment.

Calculate the percentage of oxygen in air using these results.

percentage of oxygen = ............. % 

The table shows some possible causes of anomalous results in this experiment. Use terms from the box to complete the table, showing possible causes and their effects on the volume of oxygen used in this experiment.

Each term may be used once, more than once, or not at all.

The approximate percentage of oxygen in air can be determined in the laboratory using this apparatus.

The steps for the method are shown below.

The steps shown are not in the correct order.

  Which of the following shows the correct order of the steps above?

A teacher uses the reaction between phosphorus and oxygen to calculate the percentage of oxygen in air. She uses this apparatus and excess phosphorus.

The volume of gas in the tube decreases as the phosphorus reacts with oxygen. The teacher measures the volume of gas in the tube at one-minute intervals. The table shows the teacher’s results.

State how the results show that all the oxygen has reacted.

Give one change to this experiment that would make the results more accurate.

Use the results to calculate the percentage of oxygen in air. Give your answer to one decimal place. 

percentage = ...........................%

Metal carbonates undergo thermal decomposition when heated.

Which is a correct balanced symbol equation for the thermal decomposition of a metal carbonate?

Combustion reactions involve a chemical change in which oxygen reacts with elements or compounds to produce oxides.

Which is the correct statement about combustion?

A teacher uses this apparatus to find the percentage of oxygen in a gaseous mixture of oxygen and argon.

This is the teacher’s method.

1. Heat the copper powder

2. Push the plunger on syringe A to pass the mixture of oxygen and argon over the hot copper so that the mixture moves into syringe B

3. Push the plunger on syringe B to pass the mixture of oxygen and argon over the hot copper so that the mixture moves into syringe A

4. Record the reading on syringe A

5. Repeat Steps 2, 3 and 4 a number of times

The volume of gas decreases as the oxygen reacts with the copper.

Argon is unreactive so does not react with the copper.

 The copper powder turns black. 

i) Give a reason why the copper powder is heated.

 (1)

 ii) State why argon is unreactive.

 (1)

 iii) Give the name of the black powder that forms when the oxygen reacts with the copper. 

 (1)

The table shows the teacher’s results.

i) State how the results show that all the oxygen has reacted.

 (1)

ii) The volume of gas in the glass tube and connecting tubes is 175 cm³.

 Calculate the percentage of oxygen in the mixture of oxygen and argon.

 (3)

 iii) Suggest one reason why the calculated percentage of oxygen in the mixture may not be accurate.

 (1)

A student uses this apparatus to investigate the effect of heat on different solid metal carbonates.

This is the student’s method:

• Use a spatula to put some metal carbonate in the boiling tube

• Fit the delivery tube into position

• Pour some limewater into the test tube

• Start a timer and immediately begin to heat the metal carbonate

• Record the time when a change first occurs in the limewater

The student repeats the method using different metal carbonates.

When a metal carbonate is heated a reaction sometimes occurs. The equation for the reaction is:

metal carbonate → metal oxide + carbon dioxide

State the name given to this type of reaction.

State two variables that the student should control in this investigation.

1. ........................................... 2. ...........................................

Suggest why bubbles appear in the limewater immediately after heating has started but before there is any change to the metal carbonate.

Explain the purpose of limewater in this investigation.

The table shows some of the results for the student’s investigation.

i) State the colour change that occurs for copper(II) carbonate.

from ...................................... to ...................................

(2)

ii) Give a chemical equation for this reaction of copper(II) carbonate.

(1)

f) There is a relationship between the position of a metal in the reactivity series and how easily the metal carbonate reacts when heated.

I) Use the student’s results and your own knowledge to deduce this relationship.

(2)

ii) State how you should extend the investigation to see if your deduction is correct.

(1)

This question is about determining the percentage composition by volume of a mixture of three gases, oxygen, carbon dioxide and argon in air.

Two experiments are done to determine the percentage composition by volume of oxygen.

 In experiment 1, a student connects a gas syringe to a conical flask filled with iron filings. The diagram shows the apparatus the student uses. 

 i) Give two changes the student needs to make to the apparatus in experiment 1 to find the percentage by volume of oxygen in a sample of air.

(2)

In experiment 2, two 100 cm³ gas syringes are connected to a reaction tube containing copper powder. The sample of air is passed over hot copper oxide. The diagram shows the apparatus that is used.

 ii) Explain why both syringes should not be filled with 100 cm³ of oxygen.

 (2) 

When the apparatus in experiment 2 is set up correctly, the mixture of gases is passed over the hot copper powder. 

The total volume of the mixture of gases in the syringes at the start is 76 cm³.

The total volume of the mixture of gases in the syringes at the end is 64 cm³.

 i) Calculate the percentage by volume of oxygen in the mixture of gases.

 (2)

 ii) Explain how the answer to part (i) suggests that all of the oxygen has not reacted.

 (1)

To determine the percentage of carbon dioxide in the air, the student suggests adapting the apparatus in experiment 2. 

They suggest replacing the reaction tube containing copper powder with a conical flask containing limewater, as shown.

 Explain why this experiment will not show the presence of carbon dioxide and give two reasons why this experiment will not allow the percentage of carbon dioxide in the air to be determined.

The percentage by volume of argon cannot be determined directly because it is very unreactive. 

Explain why argon is extremely unreactive.

This question is about the combustion of fuels and atmospheric pollution.

Methane is a common fuel that can be combusted.

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

Calculate the mass of methane that is required to produce 50.0 g of carbon dioxide.

An experiment is set up to show the products of combustion for methane.

 i) Write a chemical equation for the reaction that occurs in the limewater as it gives a positive test for one of the combustion products.

 (1)

 ii) The cobalt chloride paper tests for the other combustion product. Explain why the set-up of this apparatus does not prove that this combustion product is produced.

 (2) 

Fossil fuels are burned in car engines releasing carbon dioxide and other gases. Rainwater is acidic because carbon dioxide dissolves in water to form carbonic acid along with other acidic pollutant gases, such as nitrogen dioxide, dissolving in the water.

 i) Give the name of the acid that forms when nitrogen dioxide dissolves in water.

 (1)

 ii) Explain how nitrogen dioxide is produced in a car engine if there are no nitrogen-based impurities in fossil fuels?

 (2)

Explain how reducing the amount of sulfur in fossil fuels reduces the erosion of limestone.

This question is about the percentage of oxygen in the air.

The percentage of oxygen in the air can be determined by observing the formation of hydrated iron(III) oxide. 

Write the balanced chemical equation for the formation of iron(III) oxide from iron.

A student sets up this apparatus to observe the formation of hydrated iron(III) oxide.

 The reading on the syringe is recorded at the start of the experiment. The apparatus is then left for a week before the reading on the syringe is recorded again. 

The table shows the initial reading of the syringe.

The atmosphere is comprised of 20% oxygen. Complete the table to show the expected results.

A student completes a second experiment. 

The diagram shows the readings on the student's experiment.

 The student completes their results table, as shown, to calculate that the percentage of oxygen in the air is 21.5%.

Identify the error that the student has made and calculate the correct percentage of oxygen in the air.

Another set of experiments featured changes to the apparatus.

 i) Explain how the use of dry iron filings would affect the calculated percentage of oxygen in the air.

 (3)

 ii) Explain how leaving the apparatus in a warm place for 1 week would affect the calculated percentage of oxygen in the air.

 (3)