Molar Mass of a Gas
Problem: A volatile liquid is placed in a flask whose volume is 590.0 ml and allowed to boil until all of the liquid is gone, and only vapor fills the flask at a temperature of 100.0 oC and 736 mm Hg pressure. If the mass of the flask empty was 148.375g and after the experiment 149.457 g, what is the molar mass of the liquid? Plan: Use the gas law to calculate the moles, then its molar mass. Solution:
Pressure = 736 mm Hg x
1 atm 760 mm Hg
= 0.9684 atm
mass = 149.457g - 148.375g = 1.082 g ( 0.9684 atm)(0.590 L) n = (PV/RT) = (0.0821 L atm/mol K)(373.2 K)
= 0.0186 mol
M = mass / n = 1.082 g / 0.0186 mol = 58.02 g/mol note: the compound is acetone C3H6O = MM = 58g mol !