The vapour density formula expresses the ratio of the mass of a certain number of molecules of a gas to the mass of the same number of molecules of hydrogen gas. In simpler terms, it tells us how heavy a gas is compared to hydrogen gas.
Understanding Vapour Density
Vapour density is a relative measure and it is calculated as the ratio of the masses of equal numbers of molecules of the two substances. It offers a convenient way to compare the densities of gases, especially when considering gases are much less dense compared to liquids or solids.
The Vapour Density Formula Explained
The reference provided gives the formula as:
vapour density = mass of n molecules of gas / mass of n molecules of hydrogen gas
Where 'n' represents an equal number of molecules.
Let's break this down:
- mass of n molecules of gas: This is the total mass of a specific number of molecules of the gas you are analyzing.
- mass of n molecules of hydrogen gas: This is the total mass of the same number of molecules of hydrogen gas as used for the other gas. Hydrogen gas (H2) is used as the standard for comparison in vapour density calculations due to its lightness.
Practical Insights and Example
Since we are dealing with equal numbers of molecules, this means:
- The mass of 'n' molecules is proportional to the molar mass (molecular weight).
- Therefore, we can represent the vapour density as the ratio of the molar mass of the gas to the molar mass of hydrogen gas.
The molar mass of hydrogen gas (H2) is approximately 2 g/mol. Thus, using the molar masses we can rewrite the formula as:
Vapour Density = Molar Mass of the Gas / Molar mass of Hydrogen Gas
Vapour Density = Molar Mass of the Gas / 2
- Example: If you have a gas with a molar mass of 28 g/mol, the vapour density will be 28/2 = 14.
Key Points to Remember
- Vapour density is a dimensionless quantity because it is a ratio of masses.
- It is important to remember that the comparison is made for an equal number of molecules, not necessarily equal volumes.
- Vapour density provides a simple and quick way to estimate the molar mass of an unknown gas as the Molar mass of the Gas = Vapour Density * 2
