How to Calculate Partial Pressures
The partial pressure of one gas in a mix of several is the proportion of the total pressure that the one gas contributes. In other words, if you suddenly took away the other gases and left the first gas, the partial pressure is the amount of pressure it would exert. Dalton’s law of partial pressure states that the pressure exerted by a mixture of several gases equals the sum of the pressures exerted by each gas alone occupying the same volume as the mixture. For example, nitrogen gas makes up 78 percent of air, by molecular count. So if the air pressure is 14.7 pounds per square inch (PSI), then the partial pressure of nitrogren is 11.5 PSI.
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Instructions
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Convert the measure of the gases in a mixture into moles. A mole is a count of the number of molecules. In scientific notation, one mole equals about 6.022x10^23, where the caret ^ indicates exponentiation. This number is therefore approximately a 6 with 23 zeroes after it, i.e. it’s almost two thirds of a trillion trillion.
Make the conversion from mass by converting to moles as follows. Suppose your mixtures has 3 grams each of N2 and O2 and no other gas. Here the 2 is meant as a subscript referring to the number of such atoms per molecule. Look up the atomic mass units (amu) for each element. That number will also equal the number of grams per mole. Therefore N2 has 28.013g/mol and O2 has 31.999g/mol. Therefore, 3g of each means there are 0.1071 moles of N2 and 0.09375 moles of O2.
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Determine the total number of moles in the mixture.
Continuing with the example above, there are 0.1071+.09375=0.2009 moles total.
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Determine the molar proportions of each gas. For the above example, N2 has a molar proportion of 0.1071/0.2009 = 0.5331. Subtracting this from 1 gives O2’s molar proportion: 0.4669.
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Multiply the molar proportions by the total pressures.
Suppose that in the above example the total pressure is 15 PSI. Then the partial pressure of O2 is 15x0.4669=7.004 PSI. Subtracting this from 15 gives the partial pressure of N2: 7.996 PSI.
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Tips & Warnings
Note that the molar proportion of a gas equals its volumetric proportion, as discussed in Feynman’s “Lectures on Physics.” Its molar proportion is however not equal to its mass proportion, as you demonstrated in the steps above.
