A new field technician was sent out to collect a groundwater sample from a monit

Question

A new field technician was sent out to collect a groundwater sample from a monitoring well. He filled only half of the 40 mL sample vial with groundwater contaminated with benzene (T=20C). The benzene concentration in the collected groundwater was analyzed to be 5 mg/L.

a. What is the concentration of benzene in the headspace (in ppm V) before the vial was opened (1 points).

b. What is the true benzene concentration in the groundwater, if headspace free sample is collected (1 points).

(Henry's law constant for benzene is 1/5.55 mol/L

Explanation / Answer

kH = 1/5.55 mol/L-atm

p = kH*c

c= 5mg/L = 5/(1000*78.11) = 6.401*10^-5 mol/L

also volume of smple = 40ml = 0.04L

T = 293 K

so partial pressure f benzene = p = kH * C

p = 1/5.55 * 6.401*10-5 = 1.15*10-5 atm

conc in head space = p/RT = 1.15*10-5/0.082*293 = 4.786*10-7 mol/L

conc in head space in ppm = 4.786*10-7 * 1000*78.11 = 0.037 ppm..................ANS 1

so actual concentration in headspace free sample = 5-0.037 = 4.963 ppm..............ANS 2

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