9701 MCQ Solution 63

Question 103: [Organic > Hydrocarbons]

The complete combustion of alkanes to produce carbon dioxide and water is an important exothermic reaction.

Which line on the graph shows the relationship between the number of carbon atoms in the alkane and the number of moles of oxygen gas needed for complete combustion of the alkane?

Reference: Past Exam Paper –June 2003 Paper 1 Q23

Solution 63:

Answer: B.

1 mole of oxygen gas has 2 oxygen atoms. In the combustion of 1 carbon atom, 1 mole of CO₂ is formed. This requires 1 carbon atom and 2 oxygen atoms. Thus 2 carbon atoms requires 2 moles of oxygen gas (which actually has 4 atoms of oxygen), to form 2 moles of CO₂. Hence the number of moles of oxygen gas and number of carbon atoms are directly proportional, in a 1:1 ratio. This is only depicted by the line in option B. Option A and C are curved thus do not show a constant proportionality, thus are eliminated. Option D suggests the amount of oxygen gas required for the combustion of any number of carbon atoms is the same, which is illogical, thus is eliminated.