Showing that air contains about one-fifth oxygen
5 marks · HardThe diagram below shows apparatus used to investigate the amount of oxygen in air using heated copper.
Describe and explain how this experiment is carried out to determine the percentage of oxygen in air.
State what change you would expect to see in the copper during the experiment, and explain your answer.
Describe how you would use the readings from the gas syringes to calculate the percentage of oxygen in air. Include the calculation in your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Copper turns from pink-brown to black as it reacts with oxygen to form copper(II) oxide. · must show colour change described, reason given (reaction with oxygen/copper(II) oxide formation)
(b) Subtract the final total gas volume from the initial to find the oxygen used up, then (oxygen volume / initial total volume) x 100 gives percent oxygen. · must show subtract volumes to get oxygen used, divide by starting volume, multiply by 100 for percentage
Must show: copper colour change explanation, mention of forming copper(II) oxide, correct calculation using difference in volumes, reference to initial and final syringe readings
Common mistakes: Forgetting to subtract volumes; Not referring to initial/final readings
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States copper changes from pink-brown to black OR describes colour change.
B2 (a) · 1 mark · Explains the change is due to reaction with oxygen forming copper(II) oxide.
M1 (b) · 1 mark · States that the difference between initial and final gas volumes equals the volume of oxygen used.
M2 (b) · 1 mark · Describes dividing oxygen volume by initial volume to get fraction/percentage.
A1 (b) · 1 mark · Gives calculation: .
Worked solution
a) The copper will turn from pink-brown to black.
This is because the copper reacts with oxygen in the air and forms black copper(II) oxide.
b) Measure the initial total gas volume in the system before heating (add the readings from both gas syringes).
Heat the silica tube and pass air repeatedly over the copper until the reaction is finished and volume stops changing.
Measure the final total gas volume.
Volume of oxygen used = (initial total gas volume) - (final total gas volume).
Percentage of oxygen = (volume of oxygen used / initial total gas volume) × 100.