A gas sampling device containing 200 g of the sorbent Tenax is used to collect o

Question

A gas sampling device containing 200 g of the sorbent Tenax is used to collect organic impurities from the air that may be contaminated with 1,1,2-trichloro-1,2,2-trifluoroethane. At the end of the sampling, 240 L of air had been drawn through the sampling system, and the concentration of the chlorofluorocardon in the Tenax was 811 mg/kg. Assuming 100% trapping efficiency becasue no weight gain was observed in a second Tenax tube placed in series after the first, what was the contamination concentration in the aire in ppm? The ambient temperature was 18C and pressure was 750 mm Hg.

Explanation / Answer

To start, you are given a concentration relative to the amount of Tenax used (811mg per kg tenanx) . Because you know the amount of Tenax used (200g) you can calculate the mass of contaminant.

Now, the trick is to get from mass to ppm (parts per million). In this case, ppm is relative to volume of air (in Liters). The hint is given when the tell you temperature and pressure. Alot of times this means PV=nRT.. and it does here too. From the mass you calculated, you can get number of moles because you know what the compound is. After that, you plug in your values into the PV=nRT equation (make sure all of your units are correct) and solve.

(ppm in terms of volume is [mL impurities/L air])   

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