9701 MCQ Solution 1
Question 1: [Physical
> Atoms, Molecules & Stoichiometry]
A mixture of 10 cm3of methane and 10 cm3of
ethane was sparked with an excess of oxygen.
After cooling to room temperature, the residual gas was
passed through aqueous potassium hydroxide.
What volume of gas was absorbed by the alkali?
A 15 cm3
B 20 cm3
C 30 cm3
D 40 cm3
Reference: Past Exam Paper –November 2002 Paper 1 Q1
Solution 1:
Answer: C.
Methane and ethane are both hydrocarbons. When they undergo
combustion they produce carbon dioxide and water. These products were cooled to
room temperature. Carbon dioxide remains a gas and water becomes a liquid. The
residual gas, carbon dioxide, is passed through potassium hydroxide. Carbon
dioxide is a slightly acidic gas thus will react with the potassium hydroxide.
So basically the question is asking how much carbon dioxide is produced since
all of it reacts with the potassium hydroxide. 1 mole of methane has 1 carbon
atom thus will produce 1 mole of carbon dioxide.1 mole of ethane has 2 carbon
atoms thus will produce 2 moles of carbon dioxide. A total of 3 moles of carbon
dioxide will be formed. In this case since all volumes are taken at same
temperature and pressure we can take the initial volumes of methane and ethane
as the number of moles of each present. 10 cm3 of methane gives
10 moles of methane. 10 cm3 of methane gives 10 moles of
ethane. Total of 30 moles of carbon dioxide are formed which corresponds to 30
cm3 of carbon dioxide.
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