Chlorofluorocarbons such as CCl2F and CCI,F, have been linked to ozone depletion in Antarctica. In 1994, these gases were found
in quantities of 261 and 509 parts per trillion by volume (World Resources Institute, World resources 1996-97). Compute the molar concentration of these gases under conditions typical of (a) the mid-latitude troposphere (10 °C and 1.0 atm) and (b) the Antarctic stratosphere (200 K and 0.050 atm). Hint: The composition of a mixture of gases can be described by imagining that the gases are separated from one another in such a way that each exerts the same pressure. If one gas is present at very low levels it is common to express its concentration as, for example, ‘x parts per trillion by volume Then the volume of the separated gas at a certain pressure is x x 10-2 of the original volume of the gas mixture at the same pressure. For a mixture of perfect gases, the volume of each separated gas is proportional to its partial pressure in the mixture and hence to the amount in moles of the gas molecules present in the mixture.
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