Dumas Method

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Introduction
In order to identify new materials, scientists use a variety of chemical and physical methods to determine molecular masses. One of these methods includes the Dumas method for determining the molecular weight of a volatile liquid. This method, which was proposed by John Dumas in 1826, makes use of a volatile liquid (vaporizes at a relatively low temperature) and allows this liquid to be heated in a water bath to a known temperature and escape from a flask through a tiny opening (Giunta, 2003). In this situation, vapours are assumed to be obeying the Ideal Gas Law, which is PV = nRT. P is the current atmospheric temperature, measured in atmospheres, V is the volume of the flask, n is the moles of the gas and T is the
Molecular
Weight (g/mol) 30.23
Sample Calculations
∙ Methanol Solution: Trial # 2
Barometric Pressure = 765.8 mmHg
Atmospheric Pressure = 765.8 mmHg * 1 atm / 760 mmHg = 1.008 atm
Volume = 158 mL = .158 L
Mass = (Mass of Cool Flask Assembly After Heating) – (Original Mass of Flask Assembly) = (94.0701 g) – (93.8789 g) = 0.1912 g
R constant = 0.08206 L atm/K mol
Temperature = (98.8 °C + 273.15 °K) = (371.95 °K)
Molecular Weight = mRT / PV = (0.1912g)(0.08206 L atm/K mol)(371.95 °K)/(1.008 atm)(.158L) = 36.6 g/mol

∙ Percent error of Trial #2 = (Calculated Value-Accepted Value)/(Accepted Value) *100
= (36.6g/mol – 32.04 g/mol)/(32.04 g/mol) * 100
= 14.4%

∙ Percent error of Average Molecular Weight = (Calculated Value-Accepted Value)/(Accepted Value) *100
= (32.04g/mol - 23.34g/mol)/(32.04 g/mol) * 100
= 27.2%

∙ Unknown Solution (#279) : Trial #1
Barometric Pressure = 765.8 mmHg
Atmospheric Pressure = 765.8 mmHg * 1 atm / 760 mmHg = 1.008 atm
Volume = 151.5 mL = .1515 L
Mass = (Mass of Cool Flask Assembly after Heating) –