How good is your mole concept?

Try this question! (suitable for O level and IP students)

60.0 cm3 of oxygen gas reacted with 10.0 cm3 of a gaseous hydrocarbon in a closed vessel. 50.0 cm3 of gases remained when it was cooled. The gases were then passed through aqueous sodium hydroxide and 30.0 cm3 of oxygen gas was left. What is the molecular formula of the hydrocarbon?

Hint: Hydrocarbons form carbon dioxide and water during complete combustion.

The answer is C2H4. Did you solve it? Was it a lucky guess?

Here is a step-by-step guide to solving this question!

Drawing diagrams will help you simplify the question. Let the molecular formula of the hydrocarbon be CxHy.

CxHy reacts completely with excess oxygen gas to produce carbon dioxide gas and water.

Water vapour will be cooled and condensed to a liquid at room temperature, so it will not be part of the gaseous mixture.

Hence, the volume of gas remained after the complete combustion reaction consists of carbon dioxide gas and the excess unreacted oxygen gas. As the excess oxygen gas was 30 cm3, the volume of carbon dioxide gas produced was 50 - 30 = 20 cm3, which reacted with sodium hydroxide solution as it is an acidic oxide.

The volume of oxygen gas reacted was 60 - 30 = 30 cm3.

Avogadro's Law states that the volume of a gas is directly proportional to the number of moles of gas at constant temperature and pressure.

We can derive a chemical equation based on the volumes of gases involved in the combustion reaction as follows.

The volume ratio of the gases CxHy : O2 : CO2 is 1 : 3 : 2.

Since there are 6 oxygen atoms on the left hand side of the equation, we can balance the number of oxygen atoms on the right hand side of the equation by adding a 2 in front of H2O.

Therefore, x = 2 and y = 4

The molecular formula of the hydrocarbon is C2H4.

Hope you found this useful!