Molecular Weight Calculation
The average molecular weight of a mixture is calculated with the following equations:
Gases, Liquids or Solids from Mass Fractions:
Gases from Molar Fractions:
Gases from Volume Fractions:
Liquid and Solids from Volume Fractions:
Where:
xi = the mass fraction of the component i
yi = the molar fraction of the component i
vi = the volume fraction of the component i
MWi = the molecular weight of the component i
MWm = the average molecular weight
SGi = the density of the component i
EXAMPLE: Molecular Weight Calculation
A process stream consists of a mixture of three components: A, B and C. The following table provides the necessary information to calculate the average molecular weight.
Molecular Weight Calculation – Example
| Component | Molecular Weight (lb/lbmol) | Mass Fraction |
|---|---|---|
| A | 12.2 | 0.234 |
| B | 18.1 | 0.132 |
| C | 24.7 | 0.634 |
Using the equation shown above, the molecular weight of the mixture is 19.2 lb/lbmol.