Gas Laws
Gas laws are the laws that explain the relationships between the pressure, volume and temperature of gas. The laws show and explain how each of the gas property behaves when there is a change in any of the properties.
The gas laws are very important formulae for solving gas related problems.
Measurement of gas Pressure
Manometer is the instrument that is used to measure gas pressure. It consists of a u-tube that contains either water or mercury.
Diagram of manometer
.
Courtsey of www.chemistrytutorial.org
How to Use manometer
One end of the u-tube that contains mercury or water is opened to the atmosphere while the other end is connected to a gas supply. The free arm of the manometer is raised or lower until the mercury levels in both arm of the tube are at the same level. The gas supply is then opened to allow gas into the manometer.
The gas pressure on the mercury level in the left arm of the tube forces the mercury level in the right arm of the tube to rise up a certain height above that in the left arm of the tube. The difference in the height h between the mercury levels in both arm of the tube is determined.
Formula for Calculation
From the figure above, I will show you how to obtain the correct formula for calculation. You must not forget that atmospheric pressure act on the mercury surface at the open end of the tube.
From figure I,
When mercury level X is below level above Y,
Draw a horizontal line from level x at the left arm of the u-tube to cut the right arm of the u-tube and mark that point as y.
Theory of calculation:
Therefore,
Pressure at Y = atmospheric pressure + pressure due to h
Pressure at Y = Atm. Pr. + h
Now let us put it in the theory of calculation that I stated above,
Pressure at X = pressure at Y
gas pressure = atmospheric pressure + pressure due to h
gas pressure = Atm. Pr. + h
At this point, if atmospheric pressure is known and the pressure produced by h is also known, you can calculate the gas pressure by adding the two.
You must know that in some questions, you may be required to calculate the pressure produced by the liquid column h. In such questions, you would be given the data you heeded for the calculation of the pressure produced by liquid column h.
Note:
Formula for calculating pressure of liquid column of height h.
Pressure in liquid = height * density * acceleration due to gravity
Pressure of liquid = h *ρ * g
From figure II,
In figure II, the mercury levels in both arms of the u-tube are at the same level. In this case, h = 0. Therefore, it is only atmospheric pressure that is acting at point Y. Going by the theory of calculation, we will get,
Theory of calculation:
Pressure at X = pressure at Y
Pressure at X is the pressure from the gas.
Pressure at X = gas pressure.
Pressure at Y is the atmospheric pressure that is acting on the mercury surface at Y. there is no pressure due to h because h is zero.
Therefore,
Theory of calculation:
From figure III,
In figure III, the mercury level in the right arm of the u-tube is below the mercury level in the left arm of the u-tube.
Theory of calculation:
Pressure at X = pressure at Y
Pressure at X = gas pressure + pressure due to height of mercury column h
Pressure at X = gas pressure + h
Pressure at Y = atmospheric pressure.
Therefore,
gas pressure + h = atmospheric pressure
gas pressure + h = atm. Pr
You make gas pressure the subject of the formula:
gas pressure = Atm. Pressure – h
Also, in some other questions, you would be given the pressure exerted by the liquid column while in some other questions, you would be required to use the height of the liquid column as the pressure.
I believe that you can see that the concept is simple and easy to understand.
Applications
Worked Examples:
....
Gas laws are the laws that explain the relationships between the pressure, volume and temperature of gas. The laws show and explain how each of the gas property behaves when there is a change in any of the properties.
The gas laws are very important formulae for solving gas related problems.
Measurement of gas Pressure
Manometer is the instrument that is used to measure gas pressure. It consists of a u-tube that contains either water or mercury.
Diagram of manometer
. Courtsey of www.chemistrytutorial.org
How to Use manometer
One end of the u-tube that contains mercury or water is opened to the atmosphere while the other end is connected to a gas supply. The free arm of the manometer is raised or lower until the mercury levels in both arm of the tube are at the same level. The gas supply is then opened to allow gas into the manometer.
The gas pressure on the mercury level in the left arm of the tube forces the mercury level in the right arm of the tube to rise up a certain height above that in the left arm of the tube. The difference in the height h between the mercury levels in both arm of the tube is determined.
Formula for Calculation
From the figure above, I will show you how to obtain the correct formula for calculation. You must not forget that atmospheric pressure act on the mercury surface at the open end of the tube.
From figure I,
When mercury level X is below level above Y,
Draw a horizontal line from level x at the left arm of the u-tube to cut the right arm of the u-tube and mark that point as y.
Theory of calculation:
Pressure at X = pressure ar y
Pressure at X is the pressure from the gas
Pressure at X = gas pressure.Pressure at y is the atmospheric pressure that is acting on the mercury level at the open end of the u-tube and the pressure due to the height of mercury column, h.
Therefore,
Pressure at Y = atmospheric pressure + pressure due to h
Pressure at Y = Atm. Pr. + h
Now let us put it in the theory of calculation that I stated above,
Pressure at X = pressure at Y
gas pressure = atmospheric pressure + pressure due to h
gas pressure = Atm. Pr. + h
At this point, if atmospheric pressure is known and the pressure produced by h is also known, you can calculate the gas pressure by adding the two.
You must know that in some questions, you may be required to calculate the pressure produced by the liquid column h. In such questions, you would be given the data you heeded for the calculation of the pressure produced by liquid column h.
Note:
Formula for calculating pressure of liquid column of height h.
Pressure in liquid = height * density * acceleration due to gravity
Pressure of liquid = h *ρ * g
From figure II,
In figure II, the mercury levels in both arms of the u-tube are at the same level. In this case, h = 0. Therefore, it is only atmospheric pressure that is acting at point Y. Going by the theory of calculation, we will get,
Theory of calculation:
Pressure at X = pressure at Y
Pressure at X is the pressure from the gas.
Pressure at X = gas pressure.
Pressure at Y is the atmospheric pressure that is acting on the mercury surface at Y. there is no pressure due to h because h is zero.
Therefore,
Pressure at Y = atmospheric pressure.Now let us put them in the theory of calculation as we did before.
Theory of calculation:
Pressure at X = pressure at Y
gas pressure = atmospheric pressure
From figure III,
In figure III, the mercury level in the right arm of the u-tube is below the mercury level in the left arm of the u-tube.
Theory of calculation:
Pressure at X = pressure at Y
Pressure at X = gas pressure + pressure due to height of mercury column h
Pressure at X = gas pressure + h
Pressure at Y = atmospheric pressure.
Therefore,
gas pressure + h = atmospheric pressure
gas pressure + h = atm. Pr
You make gas pressure the subject of the formula:
gas pressure = Atm. Pressure – h
Also, in some other questions, you would be given the pressure exerted by the liquid column while in some other questions, you would be required to use the height of the liquid column as the pressure.
I believe that you can see that the concept is simple and easy to understand.
Applications
Worked Examples:
....
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