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 = Latm/Kmol when P in atm and V in L
P = Pressure
a. Units of Gas Pressure
i. atmosphere (atm) = in Hg = 760 mm Hg
ii. Torr = mm Hg 1 atm = 760 torr
Pascal (Pa) = 1 N/m 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