Example: If carbon monoxide is present in air at a concentration of 1 ppm, how many Co molecules are there in 1m3. and what is the mass of co in 1 m3? Solution Air at standard temperature, and pressure(STP, 0C and 1 atm)has Avogadro's number (6.02x102)of molecules per mole, which occupies 22.4 liters or 0.0224 m. Therefore the molecular density of air is 6.02x1023 molecules/0.0224 m3 2 69x1025 molecules/m3 At 1 ppm of CO, there are in 1 m3 of air (1X106)×(269×1025)=269×1019 molecules co Since Co has a molecular weight of 28, the corresponding mass density is (269×1019)x(28/602x1023)x103=1.25mg/m3Example: If carbon monoxide is present in air at a concentration of 1 ppm, how many CO molecules are there in 1m3 , and what is the mass of CO in 1 m3? Solution Air at standard temperature ,and pressure (STP, 0 oC and 1 atm ) has Avogadro’s number (6.02x1023) of molecules per mole, which occupies 22.4 liters or 0.0224 m3 . Therefore the molecular density of air is 6.02x1023 molecules / 0.0224 m3 = 2.69x1025 molecules/m3 At 1 ppm of CO, there are in 1 m3 of air (1x10-6 )x(2.69x1025) = 2.69x1019 molecules CO Since CO has a molecular weight of 28, the corresponding mass density is (2.69x1019)x(28 / 6.02x1023)x103 = 1.25 mg/m3