Partial pressure of a gas.I don't understand

[scilearner]

Hello everyone,

 

I'm confused with partial pressure of a gas.

 

Wiki: In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.

 

Now in a container if I have oxygen and carbon dioxide. If I remove carbon dioxide, shouldn't the oxygen occupy more space and its pressure decrease. So how can you call partial pressure of a gas pressure which the gas would have if it alone occupied the volume. Thanks

[timo]

The oxygen occupies the whole volume before you remove the CO2, and it does so after you remove the CO2. A gas occupying a volume does not restrict another gas to occupy the same volume (in fact it has to for the whole thing to be a mixture). Keep in mind that thermodynamics is about the large-scale effects (the "thermodynamic limit"), not about atomistic "but in reality, the atoms cannot occupy the same place at once"-details.

 

EDIT: An alternative statement applying in this special case and which more people will like: Ideal gases don't interact. So your ideal-gas oxygen doesn't feel the presence or absence of the idea-gas CO2, anyways.

Edited December 11, 2010 by timo

[cypress]

Adding to timo's ideal gas law explanation, since P=nRT/V, if you have n moles of oxygen and m moles of CO2 the ratio of partial pressures would be the ratio of molecules or the ratio of the total volume each gas occupies. So partial pressures are a convenient way of describing molecular concentration or alternatively the volume fraction of the gas phase.

 

You are correct that if you remove the CO2, the net pressure in the container would drop to equal what was the partial pressure of the Oxygen in the mixture. Returning to my previous description, if the partial pressure of Oxygen is Po, CO2 is Pc, and the total is Pt then the volume fraction of O is Po/Pt. when Po = Pt, as it would be if you removed the CO2, the volume fraction is 1 and you confirm the container is 100% Oxygen.

[scilearner]

Thanks both of you. Nice explanations

[alpha2cen]

I'm confused with partial pressure of a gas.

 

Pressure comes from gas molecule collision on the wall surface.

P_O2 can be obtained without any collision of CO2 molecules on the wall surface.

Partial pressure means the part of collide pressure on the wall surface.

Pressure = Force / area

Edited December 11, 2010 by alpha2cen

[scilearner]

Now I'm confused. Ok let's say there is a container with oxygen (Pa=100mmHg) and carbon dioxide (p=20mmHg) . Now total pressure is 120 mmHg. So if I remove carbon dioxide, the total pressure in the chamber becomes 100 mmHg. Is that right?

 

Ok but inside the lung there is oxygen and carbon dioxide. If I breathe fast and remove carbon dioxide, the partial pressure of oxygen increases? Why is that? When carbon dioxide is rapidly removed, does more oxygen from atmosphere enter the lung. Is that the reason. Please answer both questions. Thanks

[cypress]

Now I'm confused. Ok let's say there is a container with oxygen (Pa=100mmHg) and carbon dioxide (p=20mmHg) . Now total pressure is 120 mmHg. So if I remove carbon dioxide, the total pressure in the chamber becomes 100 mmHg. Is that right?

 

Yes, it is right but the context is a little off. Try to remember that although the value of the partial pressure will give you the final pressure of the container when all other kinds of molecules are removed, this is not the actual meaning of partial pressure. Partial pressure is a proxy measurement of mole fraction and/or volume fraction in a gas mixture. The reason that partial pressure equals final pressure when all other gasses are removed is because at that point volume and mole fraction equal 1 so Ptotal = pO₂.

 

Keep this always in mind ---> Partial pressure is most often not intended to measure a pressure or give you the physical property of the pressure that a gas exerts on the walls of a container. It is an indirect measure of the mole fraction of a substance in a mixture of gas in a actualized or real situation.

 

One special case of partial pressure is the (partial) pressure exerted by the vapor of a pure substance at a particular temperature in equilibrium with its liquid. Since it is pure, again the partial pressure equals the total pressure so you can empirically determine the partial pressure of a substance. This definition is useful to provide a basis for the indirect measure of mole fractions when gasses are mixed. The relationship between mole fractions and pressure at a particular temperature is an artifact of the Ideal gas law which is an artifact of mass and energy balance considerations, an outgrowth of the first law of thermodynamics and enthalpy. Total energy in a system is the energy it takes to create the substance which is the internal energy and is a direct function of its temperature plus the energy it takes to displace the environment to make room for the substance, which is pressure times volume, P*V. So if the molecules don't interact (and change internal energy in these interactions), then P*V is proportional to the quantity of the substance (n in moles) times its temperature. PV=nRT. Liquids and solids interact heavily so PV=nRT does not work for liquids and solids at all, but it does work for gasses pretty well. Ok I hope this diversion helps.

 

Don't confuse this liquid equilibrium situation with the general cases where you don't have liquid substances, but instead are just given partial pressures (really what you are given is mole fractions). Because most often the mole fractions are the items of interest in dealing with gas mixtures.

 

Ok but inside the lung there is oxygen and carbon dioxide. If I breathe fast and remove carbon dioxide, the partial pressure of oxygen increases? Why is that? When carbon dioxide is rapidly removed, does more oxygen from atmosphere enter the lung. Is that the reason. Please answer both questions. Thanks

 

Yes the partial pressure of Oxygen does increase because the partial pressure is a measure of mole fraction and in the case you describe, a dynamic state of heavy breathing, is not in equilibrium with the blood (a source of CO2) or the atmosphere (a source of O2) so when CO2 is reduced, and total pressure in the lung stays the same or nearly the same, then the mole (and volume) fractions of the other substances must go up and thus the partial pressures of the other substances (including Oxygen) will go up.

 

Edit to add clarity:

 

The situation with the lung is different from the previous situation because the container is not closed so that when CO2 is reduced, additional molecules of the other substances come into the lung to replace the reduced CO2, maintaining the total pressure at near atmospheric pressure.

Edited December 11, 2010 by cypress

[scilearner]

Thanks Cypress now I understand