Question

1. VOLUME TECHNIQUE: This method is based on the volume of carbon dioxide produced and collected.
2. MASS TECHNIQUE: This method is based on the difference in mass before and after the reaction.

The mass of carbon dioxide calculated using the volume technique will not be identical to the mass of the carbon dioxide calculated using the mass technique. In this discussion, compare and contrast the two techniques and the mass of carbon dioxide obtained by each technique. Also discuss reasons for the difference in masses.

Answer 1

Since gases have such small densities, it is usually not practical to collect the gas and find its mass. For gases that are not particularly soluble in water, it is possible to collect the evolved gas by displacement of water from a container.

The pressure inside the bottle is partially from the gas being collected and partially from the water vapor that has escaped from the surface of the water in the jar. The water inside the jar will reach an equlibruim state where the number of molecules leaving the surface is the same as the number returning. The equilibrium pressure of water is temperature dependent and is called the vapor pressure of water.

Dalton's Law of Partial Pressures tells us that the total pressure in the container must be the sum of the pressures of the gas we collected and the water vapor.

P_T= P_gas+ P_H2O

This equation can be used to calculate the pressure of the gas collected. Once the pressure of the collected gas is known, the number of moles of gas can be calculated using the ideal gas law:

PV= nRT

In mass technique the difference of the masses of the reaction mixture is taken before and after the reaction.

Since most of the pressure inside the jar is due to the water vapour, generally the volume technique produces lower value of mass of carbon dioxide.