Showing that air contains about one-fifth oxygen
3 marks · EasyDescribe how an experiment using phosphorus could be used to show the percentage of oxygen in air.
State what happens to the phosphorus when it reacts in a jar of air.
Describe how the volume of air changes during the experiment.
Explain how the result of this change can be used to find the percentage of oxygen in air.
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Show answer
(a) It burns/reacts to form phosphorus oxide. · must show burning or reacting of phosphorus
(b) The volume of air decreases. · must show decrease in air volume
(c) The decrease in volume represents the oxygen used up; calculate the percentage by (decrease in volume/original volume) × 100. · must show using volume change to calculate oxygen percentage
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that phosphorus burns or reacts with oxygen in air.
B2 (b) · 1 mark · States that the volume of air decreases as oxygen is used up.
B3 (c) · 1 mark · Explains that decrease in air volume shows the amount of oxygen, and percentage is calculated as (volume decrease/original volume) × 100.
Worked solution
a) The phosphorus burns/reacts in air, forming phosphorus oxide. b) As the phosphorus reacts, the volume of air decreases. c) The decrease in volume shows how much oxygen was present. Calculate the percentage of oxygen by (volume decrease/original volume) × 100.