Question

One of the CFCs that is implicated in decreasing the ozone layer and formation of the ozone hole over Antarctica is Freon-12 or CCl2F2. Calculate the density (in g/L) of Freon-12 at STP and compare it to the average density of air. Suggest how Freon-12 can get into the stratosphere to threaten the ozone layer.

Answer 1

For STP conditions we have temperature of 273.15 K and an absolute pressure of exactly 10⁵ Pa.

Molar mass of CCl2F2 is 121 g.

From ideal gas equation know that

PV=nRT

P:pressure, V: volume, T temperatiure, n: no. of moles.

so further

PV=nRT

PV=(w/M) RT..........w: given mass and M= molar mass

PM=(w/V)RT....w/V=density.....let it be d

PM/RT=d

putting the STP conditions in the above equation we get the density of freon-12

we have

100000 x 121/8.314 x 273.15 =5328.12 g/m3=5.328 g/L

and the density of air at STP 1.29 g/L.

So we see that the density of air is lesser than that of the freon-12.

Ozone is a highly reactive molecule that easily reduces to the more stable oxygen form with the assistance of a catalyst. Freon-12 and other Cl or Br carrying the compounds especially chlorofluorocarbons, which can travel to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br or F(in case of Freon-12) atoms are released from the parent compounds by the action of ultraviolet light, these will react with ozzone and as ozone is very reactive it will readili get converted into O2 and thus deplete the ozone layer.