Gases: The Empirical Gas Laws - Partial Pressures of Gases
When Dalton was conducting his studies, which led him to the atomic-molecular theory of matter, he also included studies of the behavior of gases.  These led him to propose, in 1803, what is now called Dalton's law of partial pressures:
For a mixture of gases in a container, the total pressure exerted is the sum of the pressures that each gas would exert if it were alone.
This law can be expressed in equation form as:   p = p1 + p2 + p3 + ...
where p is the total or measured pressure and p1, p2, ... are the partial pressures of the individual gases. For air, an appropriate form of Dalton's law would be:
p(air) = p(N2) + p(O2) + p(CO2) + ...
At temperatures near ordinary room temperature, the partial pressures of each of the components of air is directly proportional to the number of moles of that component in any volume of air.  When the total pressure of air is 100 kPa or one bar, the partial pressures of each of its components (in kPa) are numerically equal to the mole per cent of that component (Table). Thus the partial pressures of the major components of dry air at 100 kPa are nitrogen, 78 kPa; oxygen, 21 kPa; argon, 0.9 kPa; and carbon dioxide, 0.03 kPa.
The same substance may be found in different physical states under different conditions. Water, for example, can exist as a solid phase (ice), a liquid phase (water), and a gas phase (steam or water vapor) at different temperatures. The processes by which a substance is converted from one phase to another are called by specific names. The conversion from solid to liquid is melting and the reverse conversion from liquid to solid is freezing. The conversion from liquid to gas is called boiling or vaporization and the reverse conversion from gas to liquid is called condensation. The conversion from solid to gas, when it occurs directly without going through a liquid state as in the case of iodine and carbon dioxide, is called sublimation; the reverse conversion from gas to solid shares the name of condensation. 
Table: Composition of Dry Air at Sea Level
Component Mole Percent Molar Mass
N2 78.08  28.013
 O2 20.948 31.998
Ar 0.934 29.948
CO2  0.0314 44.010
Ne  0.001818 20.183
He  0.000524 4.003
CH4 0.002 16.043
Kr 0.000114 83.80
H2 0.00005 2.016
N2O 0.00005 44.013
Xe 0.0000087 131.30
Table Footnotes: The amounts of water vapor and of trace gases such as ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and ammonia in air are variable under natural conditions. Unusually high concentrations of these gases are often found in urban air as a result of human activities. The values given as mole percents in this table are numerically equal to the partial pressures of the gases, in kPa, when the total atmospheric pressure is 100 kPa or one bar.
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