Question

Benzene is a known human carcinogen. The boiling point of benzene is 80. °C (reference 1).

a) Research online to find the short term exposure limit (STEL) for benzene. What does the STEL mean?

b) A friend is planning to do an experiment with liquid benzene at 21 °C on the open bench top in a small (18 m3) poorly ventilated room (i.e. consider the room to be a closed system for this question). Convert the STEL to vapour pressure for this room. Disclaimer: You should never work with a human carcinogen outside a fumehood under any circumstances!

c) Calculate the theoretical vapour pressure of benzene inside the room at 21 °C. Use your data to explain to your friend why he can or cannot do the experiment.

d) Fully explain why the following equation (derived from the ideal gas law) cannot be used to calculate the vapour pressure/temperature data at another temperature/pressure. (P1/T1)=(P2/T2)

Answer 1

a. As per the Guidelines of NIOSH standard the STEL for benzene is 5ppm

STEL -  acceptable average exposure over a short period of time, usually 15 minutes as long as the time-weighted average is not exceeded. STEL is a term used in occupational health, industrial hygiene and toxicology.

b. the total pressure inside the room can be calculated usin the ideal gas equation .

pressure = RT/V

= (8.314 * 294) / 18

= 135.75 Pa

now the vapour pressure of benzene = 5*135.75 Pa

=678.97 Pa