The molar mass of a volatile substance was determined by the Dumas-bulb method .

Question

The molar mass of a volatile substance was determined by the Dumas-bulb method . (In the Dumas-bulb technique for determining the molar mass of an unknown liquid, you vaporize the sample of a liquid that boils below 100 degree C in a boiling-water bath and determine the mass of vapor required to fill the bulb (see drawing ).) The unknown vapor had a mass of 0.845 g ; the volume of the bulb was 354 cm^3 , pressure 753 torr . and temperature 100 degree C. Calculate the molar mass of the unknown vapor.

Explanation / Answer

Using ideal gas equation

PV = nRT

P = 753 torr = 0.991 atm

V = 354 cm^3 = 0.354 L

R = 0.0821 L atm / mol K

T = 100 degreeC = 373 K

PV = nRT

n = PV / RT

n = (0.991 atm * 0.354 L) / ( 0.0821 L atm / mol K * 373 K)

n = 0.0114 mol

Moles = given mass / molar mass

Molar mass = Given mass / moles = 0.845 g / 0.0114 mol = 74.12 g/mol

Molar mass of unknown vapor = 74.12 g/mol

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