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Question

In the process of identifying an unknown liquid it was necessary to determine it's molecular weight. The Dumas method was chosen for this measurement. It was found that at a temperature of 98°C and at a pressure of 743 Torr, 1.220 g of vapor occupied 375 mL. Determine the molecular weight of the unknown

Answer 1

Molecular weight (molar mass) = grams / moles

We know the grams = 1.220 g, so we need to find the moles. This is done using the ideal gas law:

PV = nRT

P = pressure in atm = 743 torr x 1 atm / 760 torr = 0.978 atm

V = volume in liters = 375 ml x 1 L / 1000 ml = 0.375 L

n = moles = ?

R = gas constant = 0.0821 Latm/Kmol

T = temperature in Kelvin = 98ºC + 273 = 371K

Solving for n...

n = PV/RT = (0.978)(0.375) / (0.0821)(371)

n= 0.01204 moles

Molecular weight = 1.220 g / 0.01204 moles = 101.4 g / mole