Vapour density is a term used to describe the density of a gas or vapor relative to the density of a reference gas, typically air. It is commonly expressed as the ratio of the molecular weight of the vapor to the molecular weight of the reference gas.
To calculate the vapor density, you can follow these steps:
Determine the molecular weight of the vapor: The molecular weight of a substance is the sum of the atomic weights of all the atoms in its chemical formula. You can find the atomic weights from the periodic table.
Determine the molecular weight of the reference gas: In most cases, air is used as the reference gas. The molecular weight of air is approximately 28.97 grams per mole.
Calculate the vapor density: Divide the molecular weight of the vapor by the molecular weight of the reference gas.
Vapor Density = (Molecular Weight of Vapor) / (Molecular Weight of Reference Gas)
For example, let's say we want to calculate the vapor density of water vapor (H2O) compared to air.
Molecular weight of water (H2O) = 2(Atomic weight of hydrogen) + Atomic weight of oxygen = 2(1.008 g/mol) + 16.00 g/mol = 18.02 g/mol
Molecular weight of air (reference gas) = 28.97 g/mol
Vapor Density = (Molecular Weight of Water Vapor) / (Molecular Weight of Air) = 18.02 g/mol / 28.97 g/mol ≈ 0.622
Therefore, the vapor density of water vapor compared to air is approximately 0.622.
Note that the vapor density provides a relative measure of the density of the vapor compared to the reference gas. It is useful for comparing the densities of different vapors, but it does not provide the absolute density of the vapor.