Question

Acrolein (propenal) is an important industrial compound and used as a contact herbicide. It has a vapour pressure of 259.8 Torr at 25.0 °C. The occupational exposure limit is 100 ppbv over 8 hours. A major spill of acrolein occurs in a closed room without ventilation at 25.0 °C and 1.00 atm. Determine the mixing ratio of acrolein in the room air (by volume) after steady state is reached.

Answer 1

Step1 Convert torr into atm

Patm = Ptorr / 760

259.8 Torr = 259.8 /560 = 0.34 atm

Step 2 Calculate the mole fraction of acrolein

Dalton's Law states, that total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases.

P_T = P₀ + P_ac

P_T = 1 atm + 0.34 atm = 1.34 atm

The partial pressure of any gas in a mixture is the total pressure multiplied by the mole fraction of that gas.

So, mol fraction of acrolein

Xac = 0.34/1.34 = 0.254

Step 3 Calculate the mixing ratio of acrolein in the room air (by volume) after steady state is reached.

Molar mass of acrolein = 56.06 g/mol 0.1mg/l

Volume of 1 mol air at 25.0 °C and 1.00 atm = 24.43L

Volume of 1 mol contaminated air = 24.43L/1.34 = 18.23 L

The mixing ratio of acrolein in the room air (by volume)

= (0.254 mol acrolein /1.0 mol air) x (56.06 g/1mol) x (1molair/18.23L) x (10^3L/m^3) x 10^6µg/1g

= 7.69 x 10^8µg /m^3

The mixing ratio of acrolein in the room air (by volume) = 7.69 x 10^8µg /m^3