Showing that air contains about one-fifth oxygen
4 marks · NormalThe diagram shows an apparatus used to investigate the percentage of oxygen in air using wet iron filings.
Describe the steps taken to set up and carry out this experiment.
Suggest how the total volume of air present in the apparatus at the beginning can be measured.
Explain why water is used when adding the iron filings.
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.
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(a) Place wet iron filings in the flask, connect to gas syringe, record initial volume, leave several days, record final volume. · must show Add wet iron filings, initial and final readings, leave for reaction
(b) Fill apparatus with water and measure volume required to fill it. · must show Fill with water to measure volume
(c) Water is needed because iron requires both water and oxygen to rust. · must show water necessary for rusting
Common mistakes: Missing out waiting period; Not mentioning water enables rusting
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes setting up apparatus with wet iron filings, records initial reading.
M2 (a) · 1 mark · Leaves for several days, records final reading on syringe.
B1 (b) · 1 mark · Suggests filling apparatus with water to determine starting total volume of air.
B2 (c) · 1 mark · Explains water is required as iron needs water and oxygen to rust.
Worked solution
a) Add damp iron filings to the flask, connect to gas syringe, record the initial gas syringe volume, leave the apparatus for about a week, then record the final volume.
b) To find the starting volume of air, fill the apparatus with water and measure how much water is needed to fill it to the marks.
c) Water is needed because rusting (iron oxide formation) only occurs when both air (oxygen) and water are present.