9cOrganic wastes can be used as an alternative energy source. Under suitable conditions, the wastes

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• 9c)Organic wastes can be used as an alternative
  energy source. Under suitable conditions, the
  wastes can be digested by bacteria to give a
  gaseous mixture containing a high proportion of
  methane. Methane can be used as a fuel.
• Suggested ONE organic waste that can be used for
  this purpose.
• Write the chemical equation for the complete
  combustion of methane.
• Suggest ONE advantage of using organic wastes as
  an alternative energy source. (3 mins)

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• Suggest ONE reason why organic wastes are not yet
  widely used as an energy source. Total 4 mins
• Suggested Solution
• Manure/ dead plant/ dead animal/ sewage sludge.
• CH4 2O2 ? CO2 2H2O
• Save fossil fuels/ as a renewable energy source/
  reduce solid waste.
• Difficult to collect large amount of organic
  waste./
• High cost to build a biogas plants.

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• 9b) Brass is alloy consisting mainly of copper
  and metal X. It is used to make musical
  instruments such as trumpet.
• What is X?
• Suggest ONE reason for using brass instead of
  copper in making trumpets.
• In an experiment, a piece of brass is added to a
  test tube containing 2 M nitric acid.
• State TWO observations in the experiment and
  write the relevant chemical equation(s). (6 mins)

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Suggested solutions

• Zinc.
• Brass is more corrosion resistant than copper. /
• Brass is more appealing than copper.
• iii) A blue solution is formed.
• The alloy dissolves.
• A colourless gas (NO) is evolved. / Brown gas
  (NO2).
• 3Cu 8HNO3 ? 3Cu(NO3)2 2NO 4H2O
• 2NO O2 ? 2NO2

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8b) An experiment was carried out to determine
the concentration of a nickel(II) sulphate
solution. The experiment consisted of the
following three stages Stage 1 25.0 cm3 of
0.503 M sodium hydroxide solution was added to
25.0 cm3 of the nickel(II) sulphate solution to
precipitate out nickel(II) hydroxide. Stage
2 The mixture obtained in Stage 1 was filtered
and the residue was washed thoroughly with
distilled water.
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• 8b)Stage 3 The excess alkali in the filtrate was
  titrated against 0.251 M hydrochloric acid with
  methyl orange as indicator. 18.5 cm3 of the acid
  was required to reach the end-point.
• Write the ionic equation for the reaction in
  Stage 1. (1 min)
• State the colour change at the end-point of the
  titration in stage 3. (1 min)

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• Stage 3 The excess alkali in the filtrate was
  titrated against 0.251 M hydrochloric acid with
  methyl orange as indicator. 18.5 cm3 of the acid
  was required to reach the end-point
• 1)Based on the titration result in Stage 3,
  calculate the number of moles of hydroxide ions
  present in the filtrate.(2mins)
• 2) Calculate the number of moles of sodium
  hydroxide that was added in Stage 1.(1 min)
• 3) Using your results in 1) and 2) above,
  calculate the molarity of the NiSO4(aq). (3mins)

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• Ni2 2OH- ? Ni(OH)2 (1 M)
• Yellow to orange. (1M)
• 1) H OH- ? H2O (1 M)
• no. of mol of OH- no. of mol of H
• 0.251 x 18.5 x 10-3
• 4.64 x 10-3 mol (1 M)
• No. of mol of NaOH used
• 0.503 x 25 x 10-3
• 0.0126 x 10-3 (1 M)
• no. of NaOH that has reacted with Ni2
• 0.012575 4.6435 x 10-3 7.93 x 10-3 (3M)
• Ni2 7.93 x 10-3 / (2 x 25 x 10-3) 0.158 M

Suggested solutions