Gases in the Atmosphere
5 marks · EasyThis question is about experiments to show the amount of oxygen in air, and the combustion of elements in oxygen.
Describe how an experiment using wet iron filings can be used to find the approximate percentage of oxygen in air. State one precaution needed for an accurate result.
A student burns magnesium ribbon in pure oxygen. State the chemical name and appearance of the solid product formed.
Explain why elements burn more brightly in pure oxygen than in air.
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(a) — · must show description of apparatus with wet iron filings and gas syringe or measuring cylinder, oxygen reacts over time, need for excess iron or sufficient time for complete reaction
(b) magnesium oxide; a white solid · must show magnesium oxide, white solid
(c) Pure oxygen provides more oxygen than air, so elements react faster/more vigorously/with brighter flame than in air, which contains less oxygen. · must show air contains less oxygen than pure oxygen, more vigorous/bright combustion in pure oxygen
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Marking points
B1 (a) · 1 mark · Describes method: iron filings (wet) left in air/gas syringe/flask, oxygen removed as iron rusts, measure change in volume.
B1p (a) · 1 mark · States a correct precaution (e.g. use excess iron filings or leave for enough time for all oxygen to react).
B1b (b) · 1 mark · Names correct chemical (magnesium oxide) and its appearance (white solid powder).
B1c1 (c) · 1 mark · States that pure oxygen contains more oxygen than air, or air only 21% oxygen.
B1c2 (c) · 1 mark · Links increased oxygen to more rapid/brighter/vigorous combustion.
Worked solution
a) Wet iron filings are placed in a conical flask, which is connected to a gas syringe or has its volume measured. Over several days, the iron reacts with oxygen in the air to form rust (iron oxide). The decrease in gas volume shows how much oxygen has been used. For an accurate result, use excess iron filings or leave the experiment until the gas volume stops decreasing.
b) The solid product is magnesium oxide, a white solid
c) Pure oxygen contains much more oxygen (100%) than air (about 21%), so elements have more oxygen to react with. Combustion is more vigorous and brighter in pure oxygen than in air.