Chemistry 9701/52 — October/November 2012
Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme
Topics Analysis, Conclusions and Evaluation · Planning
There are three oxides of lead, PbO, PbO₂ and Pb₃O₄, all of which can be reduced to metallic lead by hydrogen.
The following information gives some of the hazards associated with these compounds.
Lead oxides
Lead(II) oxide (PbO) Lead(IV) oxide (PbO₂) Dilead(II) lead(IV) oxide (Pb₃O₄)
Toxic Dangerous for the environment
Harmful by inhalation and if swallowed. Danger of cumulative effects.
Hydrogen Extremely flammable. Readily forms an explosive mixture with air. Mixtures between 4 and 74% by volume are explosive.
An unknown sample of an oxide of lead can be identified by investigating the molar ratio of oxygen atoms to lead atoms.
You are to plan an experiment to investigate the molar ratio of oxygen atoms to lead atoms in the oxide sample. Your plan should result in a correct identification of the oxide.
Calculate the number of moles of oxygen atoms that combine with one mole of lead atoms in each of the three oxides.
Draw a sketch graph to show how the number of moles of oxygen atoms varies with the number of moles of lead for lead(II) oxide, PbO. Draw two more sketch graphs to show this relationship for the other two oxides.
Label clearly each axis and each graph.
In the experiment you are about to plan, identify the following.
the independent variable
the dependent variable
Draw a diagram of the apparatus and experimental set up you would use to determine the chemical formula of the oxide. Your apparatus should use only standard items found in a school or college laboratory and should show clearly
(i) how the hydrogen gas needed for the reduction is prepared, naming the chemicals (reagents) to be used,
(ii) how the oxide of lead will be heated,
(iii) how any excess hydrogen is dealt with safely.
Label each piece of apparatus used.
Using the apparatus shown in (d), design a laboratory experiment which will enable you to determine the chemical formula of the oxide.
Give a step-by-step description of how you would carry out the experiment by
(i) stating a suitable mass of the oxide of lead,
(ii) stating how you would ensure that the decomposition is complete,
(iii) showing by calculation the minimum volume of hydrogen, measured at , that would be needed to reduce the mass of oxide of lead stated in (i) above. For calculation purposes, you may assume that the oxide of lead is PbO,
(iv) stating how you would use your results to reach a conclusion.
State one hazard that must be considered when planning the experiment and describe a precaution that should be taken to minimise the risk from this hazard.
Draw up a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to determine graphically the formula of the oxide. The headings should include appropriate units.
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