Molar Mass by the Dumas MethodA. The Ideal Gas Law

1. PV = nRT is called the Ideal Gas Law2. R is called the gas constant

a. The value of R depends on the units of P and Vb. Generally use R = 0.08206Latm/Kmol when P in atm and V in L

3. 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

iii. Pascal (Pa) = 1 N/m2 1 atm = 101,325 Pa

4. V = Volume in Liters

5. T = Temperature in Kelvins

6. n = moles of the gas

7. Example: How many moles of H2 gas if: 8.56L, 0 oC, 1.5atm?

mol57.0

273KKmol

m0.08206Lat

8.56L1.5atm

RT

PVnnRTPV

B. The Dumas Method of Determining Molar Mass

1. Modifying the Ideal Gas Law

2. We will experimentally find g, T, P, V; and we know R: find g/mol

3. Procedure

a. Add 5-10 ml of cyclohexane to a 125 ml erlenmeyer flask

b. Cover the flask with aluminum foil; poke a small hole in the foil

c. Heat the flask in boiling water (400ml beaker) in the hood on hotplate

d. When the liquid has just vaporized, cool in an ice bath, dry, weigh

e. When finished, fill flask with water and weigh for V (dH2O = 1.00g/ml)

PV

gRT

mol

gRT

molgg

nRTPV

C. Safety

1. Cyclohexane and the unknown are flammable-hotplates, not bunsen burner

2. Fumes may be irritating—do the heating in the hood

D. How well did you do?

1. Percent Error

2. Repeat the experiment for an unknown

100%84.2

84.2-Exp%Error x

Incident: Organic Solvent Fire