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How do I find partial pressures if I am only given their percentages? Where do I begin?

Example: Assume $\ce{O}=21\%$ , $\ce{N}=78\%$ , $\ce{Ar}=1\%$ by volume. How would I calculate the partial pressures if total pressure is $\pu{1 atm}$.

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Pressures of gasses are additive. For example, 1 atm X plus 1 atm Y makes 2 atm total pressure. Because all the components of a mixed gas are at the same temperature and pressure, the fraction of each component by volume is also the same fraction of the total pressure.

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    $\begingroup$ Is this generally true or only for ideal gasses?! $\endgroup$ – Michiel Apr 28 '13 at 11:12
  • $\begingroup$ Yes, but it works quite well in any "normal" range of densities. $\endgroup$ – Paul J. Gans Apr 29 '13 at 2:18

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