Several decades ago benzene was thought to be a harmless

Chapter 5, Problem 5.22

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Several decades ago benzene was thought to be a harmless chemical with a somewhat pleasant odor and was widely used as a cleaning solvent. It has since been found that chronic exposure to UN. 1. Sax and R. 1. Lewis, Hazardous Chemicals Desk Reference, Van Nostrand Reinhold, New York. 1987, p.183. 13From D. A. Crowl. D. W. Hubbard, and R. M. Felder, Set: Stoichiometry, Center for Chemical Process Safety, New York. 1993. 218 Chapter 5 Single-Phase Systems benzene can cause health problems such as anemia and possibly leukemia. Benzene has a permissible exposure level (PEL) of 1.0 ppm (part per million on a molar basis, equivalent to a mole fraction of 1.0 X 10-6) averaged over an 8-hour period. The safety engineer in a plant wishes to determine whether the benzene concentration in a laboratory exceeds the PEL. One Monday at 9 a.m., 1 p.m., and 5 p.m., she collects samples of room air (33C, 99 kPa) in evacuated 2-liter stainless steel containers. To collect a sample she opens the container valve, allows room air to enter until the container pressure equals atmospheric pressure, and then charges clean dry helium into the container until the pressure reaches 500 kPa. Next, she takes the containers to an analytical laboratory in which the temperature is 23C, leaves them there for a day, and then feeds gas from each container to a gas chromatograph (GC) until the pressure in the container is reduced to 400 kPa. In the order in which they were collected, the samples that pass through the GC are found to contain 0.656 j.Lg (microgram), 0.788j.Lg, and 0.910 j.Lg of benzene, respectively. (a) What were the concentrations of benzene (ppm on a molar basis) in the original room air at the three collection times? (Assume ideal gas behavior.) Is the average concentration below the PEL? (b) Why did the engineer add helium to the container after collecting the room air sample? Why did she wait a day before analyzing the container contents? (c) Why might a finding that the average benzene concentration is below the PEL not necessarily mean that the laboratory is safe insofar as exposure to benzene is concerned? Give several reasons, including possible sources of error in the sampling and analysis procedure. (Among other things, note the day on which the samples were taken.)