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Molar Mass by the Dumas Method A. The Ideal Gas Law PV = nRT is called the Ideal Gas Law

Molar Mass by the Dumas Method A. The Ideal Gas Law PV = nRT is called the Ideal Gas Law R is called the gas constant The value of R depends on the units of P and V Generally use R = 0.08206Latm/Kmol when P in atm and V in L P = Pressure a. Units of Gas Pressure

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Molar Mass by the Dumas Method A. The Ideal Gas Law PV = nRT is called the Ideal Gas Law

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  1. Molar Mass by the Dumas Method • A. The Ideal Gas Law • PV = nRT is called the Ideal Gas Law • R is called the gas constant • The value of R depends on the units of P and V • Generally use R = 0.08206Latm/Kmol when P in atm and V in L • P = Pressure • a. Units of Gas Pressure • i. atmosphere (atm) = 29.92 in Hg = 760 mm Hg • ii. Torr = mm Hg 1 atm = 760 torr • Pascal (Pa) = 1 N/m2 1 atm = 101,325 Pa • V = Volume in Liters • T = Temperature in Kelvins • n = moles of the gas • Example: How many moles of H2 gas if: 8.56L, 0 oC, 1.5atm?

  2. The Dumas Method of Determining Molar Mass • Modifying the Ideal Gas Law • We will experimentally find g, T, P, V; and we know R: find g/mol • Procedure • Add 5-10 ml of cyclohexane to a 125 ml erlenmeyer flask • Cover the flask with aluminum foil; poke a small hole in the foil • Heat the flask in boiling water (400ml beaker) in the hood on hotplate • When the liquid has just vaporized, cool in an ice bath, dry, weigh • When finished, fill flask with water and weigh for V (dH2O = 1.00g/ml)

  3. Safety • Cyclohexane and the unknown are flammable-hotplates, not bunsen burner • Fumes may be irritating—do the heating in the hood • How well did you do? • Percent Error • Repeat the experiment for an unknown

  4. Incident: Organic Solvent Fire

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