The Dumas Method is a simple procedure to determine the molar mass of volatile l

Question

The Dumas Method is a simple procedure to determine the molar mass of volatile liquid. In such an experiment, a precisely known amount of the volatile liquid is transferred from the liquid state to the gas phase, and the ideal gas law in combination with the mass of the gas provides an easy access to the molecular weight of the volatile liquid. To illustrate this, consider the determination of the molar mass of diethyl ether. The following set of data was collected in a Dumas experiment: Calculate the molar mass of diethyl ether from this data. The success of this simple method depends on a lot of things going right, since it is based on various assumptions. Explain which assumptions are made when applying the Dumas Method to determine the molecular weight of a volatile liquid. The molecular weight then allows one to determine the density of the gas vapor. Use data of the example given above, and calculate the density of gaseous diethyl ether at atmospheric pressure and the temperature of boiling water.

Explanation / Answer

Duma's method use Ideal gas law to determine molecular mass

PV = nRT

Mass of condensed liquid = 135.886 - 135.263 = 0.623 g

volume of flask = 262.2 ml = 0.2622 L

P = 756.2 mmHg = 0.995 atm

T = 99.6 oC = 99.6 + 273 = 372.4 K

n = moles = g/molar mass

Feed values,

n = 0.623/molar mass = 0.995 x 0.2622/0.08205 x 372.4

molar mass of liquid = 73.0 g/mol

The sucess of Duma's method is based on the assumption that th liquid behaves ideally and the volume does change appreciably at room and higher temperature.

density of gaseous diethyether = 0.955 x 73/0.08205 x 372.4 = 2.28 g/L

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