Experiment 5: Gas Law Apparatus.

Objective: to determine how the pressure changes depending on the volume maintaining the temperature as constant as possible.

Background: 

There are three main gas laws which are combined to form the ideal gas equation. One of these three laws is applied in this experiment. But , first of all, what is an ideal gas?

An ideal gas is a hypothetical gas formed by identical particles of zero volume, with no intermolecular forces.

Charles' - Gay Lussac law: in this one, we maintain the pressure constant and we change the temperature and volume. "At constant pressure, the volume of a fixed mass of gas is a linear function of temperature".

Gay - Lussac's law: in this one, we maintain the volume constant and we change the temperature and pressure. "At constant volume, the pressure of a given mass of gas is a linear function of temperature"

Boyle - Mariotte's law: this is the one used in the experiment. In this one, we maintain the temperature constant and we change the volume and pressure. "For a fixed mass of an ideal gas at cosntant temperature, the product of pressure and volume is a constant".

Variables:

Independent variable: the independent variable in this experiment is the volume of air we put into the gas container because we get to chose how the amount of gas we want to have. We measure this variable in mL.

Dependent variable: the dependent variable in this experiment is the pressure as it changes depending on the changes in the volume. We measure this variable in hPa.

Control variable: in this case, we were told to maintain the temperature as stable as possible and so we kept it within 21ºC and 21,5ºC. This variable is measured in ºC. Another one is the atmospheric pressure (as it is the pressure we always start with). This variable is measured in hPa. The initial amount of air we have (65 mL if we want to test the positive pressure values and 20 mL if we want to test the negative ones).

Materials:

- Gas Law Apparatus.

- Computer.

Procedure:

1.- Open an Excel file on your computer. Prepare three columns: volume (measured in mL*), pressure (measured in hPa*) and temperature (measured in ºC*).

* Al these units are the ones shown in the Gas Law Apparatus. If these are not the units you have you might need to change them.

In order to obtain positive pressure values:

2.1-  Turn the clamping handle until it reaches the 65 mL.

3.1- Open the crank handle so that air, composed mainly of oxygen but mostly of nitrogen, can come in.

4.1- Once you have the 65 mL of air, close the crank handle.

5.1- Start turning the clamping handle and stop every 2 mL, in order to measure the results as accurately as possible.

6.1- As said before, stop every 2 mL and write down the corresponding volume, pressure and temperature.

7.1- Make a table in the Excel file with the corresponding data.

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In order to obtain negative pressure values:

2.2.- Turn the clamping handle until it reaches the 20 mL though it should already be there.

3.2.- Open the crank handle so that air can come in.

4.2.- Once you have the 20 mL of air, close the crank handle.

5.2.- Start turning the clamping handle and stop every 2 mL, writing down the corresponding volume, pressure (remember that you should obtain negative values) and temperature.

6.2.- Make a table in the Excel file with the corresponding data.

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8.- Repeat these two procedures as many times as you can so that you achieve a better result.

9.- In order to process the data, make two tables, one for the positive pressure values and another one for the negative ones, and the two corresponding graphs**.

** In order to obtain a lineal function you have to change the volume variable into the inverse of the volume (1/volume).

Processing Data:

Table 1: table showing the relation between the volume (measured in mL) and the pressure (measured in hPa) of air starting with an initial volume of 65 mL.

Table 2: table showing the relation between the volume (measured in mL) and the pressure (measured in hPa) of air starting with an initial volume of 20 mL.

Table 3: table showing the relation between the inverse of the volume of table 1 (1/volume) and the pressure (measured in hPa) of air.

Table 4: Table showing the relation between the inverse of the volume of table 2 (1/volume) and the pressure (measured in hPa) of air.

Graph 1: graph showing the lineal relation between the inverse volume of table 3 (1/volume) and the pressure (measured in hPa) of air.

Graph 2: graph showing the lineal relation between the inverse volume of table 4 (1/volume) and the pressure  (measured in hPa) of air.

Video 1: what is and how to use the gas law apparatus.

Video 2: why do we obtain negative pressure values?

Video 3: three gas laws we should know in order to fully understant this experiment.

Video 4: Gas Properties programme.

Conclusion:
The experiment we carried out actually served as a demonstration of Boyle's-Mariotte Law which stated that for a fixed mass of an ideal gas at a constant temperature, the product of pressure and volume is a constant (P·V=K) .
We figured this throughout the experiment and we could also see it in the tendency line represented in the graph where we obtained a regression coefficient of 0,983.
Finally, during this lab session we tried to carry out an adiabatic process, which is why we attempt to keep the heat be constant.

References:

Hyperphysics.phy-astr.gsu.edu (1998). Ideal Gas Law. [online] Retrieved from: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html