Exp. 18 Objectives

1
Exp. 18 Objectives

• To measure the physical properties of pressure,
  volume, and temperature for a gaseous substance.
  
• To determine the molar mass (molecular weight) of
  a volatile liquid.

2
Exp. 18 Introduction

• To identify a newly synthesized compound, a
  chemist must determine its physical properties
• Melting point
• Color
• Density
• Boiling point
• Molar mass

3
Exp. 18 Introduction

• Measuring Molar Mass
• Large, non-volatile compounds such as
  biomolecules pharmaceuticals
• Old osmometer
• New mass spectrometer (m/z ratio)
• Volatile liquids
• Dumas method a known mass of the unknown liquid
  is vaporized into a known volume at a measured
  temperature and barometric pressure.

4
Exp. 18 Introduction

• Using the data and the ideal gas law equation,
  the number of moles of the vaporized liquid are
  determined
• n PV/RT R 0.08206Latm/molK
• Once the number of moles (n) is known, the molar
  mass is calculated by dividing the measured mass
  (g) by n
• molar mass mass(g)/n(moles)

5
Exp. 18 Introduction

• In the real world, few gases behave ideally.
• Gases with large intermolecular forces and large
  molecular volumes do not behave according to the
  ideal gas law equation. In this case, we use van
  der Waals equation which is used to correct for
  non-ideal behavior
• (P n2a/V2)(V-nb) nRT
• a and b are called van der Waals Constants.
  They are determined experimentally and can be
  found in reference handbooks.

6
Exp. 18 Procedure Notes

• Part A Use a hot plate
• Part B.1 Adjust the water level high on the
  Erlenmeyer flask.
• Part B.2 Keep heating for 5 minutes after the
  liquid appears to be gone.
• Part B.4 Two trials, not three
• Part C.1 Check out a 150 or 200 mL graduated
  cylinder from the stockroom.

7
Exp. 18 Report Sheet

• Questions 2, 4