Question

Dumas Experiment In the Dumas experiment, a pinholed flask apparatus has an empty mass of 216.422 g. When it’s completely filled, it has a volume of 268.5 mL. It’s filled with a few milliliters of a volatile solvent which is allowed to completely evaporate when immersed in a boiling water bath at 100.9oC. Upon removal and cooling to room temperature, the flask mass has increased to 217.504 g. The atmospheric pressure is 29.97 inches of Hg. 11. Reexpress the temperature of the water bath in units of Kelvin. 12. Reexpress the atmospheric pressure in units of atmospheres. (1.000 atm = 29.92 inches of Hg) 13. Reexpress the volume of the apparatus in units of liters. 14. Calculate the moles of gas contained within the flask at the water bath temperature. 15. Use the above information to calculate the molar mass of the gas. 16. Suppose that only half of the height of the flask is immersed in boiling water. No solvent condenses at the top. In which direction will this throw off the final result for the molar mass? Explain why.

Answer 1

Dumas method for determination of molar mass

11. Temperature of water bath = 100.9 oC

                                                  = 100.9 + 273 = 283.9 K

12. atmospheric pressure = 29.97 inch Hg

= 29.97 inch Hg/29.92 inch Hg

= 1.00 atm

13. Volume of apparatus = 268.5 ml

= 0.2685 L

14. moles of gas in the flask = pressure x volume/gas constant x temperature

= 1.00 x 0.2685/0.08205 x 283.9

= 0.0115 mol

15. mass of gas = 217.504 - 216.422 = 1.082

molar mass of gas = 1.082 g/0.0115 mol = 94.1 g/mol

16. If only half of the height of the flask was immersed in boiling water, all of liquid will not be evaporated. Mass of gas would be lower. So molar mass calculated would be also lower than actual value.