Question

Prelab question for Determination of the Universal Gas Constant, R:

How does collecting the oxygen over water affect the volume of gas collected? Why does this happen?

Please be specific and answer this question directly. I've already asked once and received a general answer not specifically regarding how this affects the Volume and Why it happens.

Answer 1

Answer:

In many cases , the amount of gas evolved by a reaction is of interest. 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 Setup for the collection of gas over water involved a container in which the reaction take place and a gas collection container filled with water and inverted in reservior of water. The gas evalved from the reaction is collected by attaching one end of a hose to the reaction container and intersting the other up into the inverted gas collection bottle.

The volume of gas can be determined by the amount of water that was displaced by the gas.

The volume of gas collected and the gas laws can be used to calculate the number of moles of gas collected.

During the collection, the water level in the container will be adjust sothat the pressure inside and outside the container are the same. Because of this , if we knowthe atmospheric pressure, we also know the pressure of the gas inside the bottle.

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 water in the jar. The water inside the jar will reach an equilibrium state where the number of molecules leaving the surface is the same as the number returning. The equilibrium pressure of water is temperature dependant 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 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

Where,

P= Pressure of the gas

V= Volume of water displaced

n= number of moles of gas

R= the ideal gas constant

T= the temperature of gas