The molecular mass of a substance can be directly calculated from its vapour density using a simple relationship: Molecular mass = 2 x vapour density.
This relationship stems from Avogadro's Law and the understanding of molar volumes of gases. Here's a breakdown of how it works:
Understanding Vapour Density and Molecular Mass
- Vapour Density: Vapour density is defined as the ratio of the mass of a certain volume of a gas to the mass of the same volume of hydrogen, both measured at the same temperature and pressure. Essentially, it tells you how many times heavier a gas is compared to hydrogen under the same conditions.
- Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all the atoms in the molecule. It represents the mass of one molecule of that substance.
The Relationship Explained
The relationship between vapour density and molecular mass can be expressed by the following formula:
Molecular Mass = 2 x Vapour Density
This equation is derived from the fact that one mole of any gas occupies the same volume at the same temperature and pressure (Avogadro's Law). Since hydrogen (Hâ‚‚) has a molecular mass of approximately 2, the vapour density, when multiplied by 2, directly yields the molecular mass of the gas in question.
Practical Application
Here's how you can use this formula:
- Determine the Vapour Density: You'll need to either be given the vapour density of the substance or experimentally determine it through various laboratory techniques.
- Apply the Formula: Simply multiply the obtained vapour density by 2.
- Result: The result will be the molecular mass of the substance.
Example:
Let's say the vapour density of a gas is 15. Then, the molecular mass of the gas would be:
Molecular mass = 2 x 15 = 30
Summary
| Concept | Definition |
|---|---|
| Vapour Density | Ratio of gas mass to hydrogen mass at same temperature and pressure |
| Molecular Mass | Sum of atomic masses of all atoms in a molecule |
| Relationship | Molecular Mass = 2 x Vapour Density |
By understanding this simple relationship, you can easily determine the molecular mass of a gas when its vapour density is known. This is a fundamental concept in chemistry and crucial for various calculations and analysis.
