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.