# Methods of Determination of Molecular Mass

Following methods are used to determine molecular mass,

(i) **Diffusion method **(For gases)**: **The ratio of rates of diffusion of two gases is inversely proportional to the square root of their molecular masses.

**r _{1}/r_{2 }= √m_{2}/m_{1 }**

(ii) **Vapour density method **(For gases only): Mass of a fixed volume of the vapour is compared with the mass of the same volume of hydrogen under same conditions. The ratio of these masses is called** Vapour density** or **Relative density**.

**Molecular mass = 2×Vapour desity **

(iii) **Victor Meyer method (**For volatile liquids or solids)

It is based on Dalton's law of partial pressure and Avogadro's hypothesis (gram molar volume).

22400 ml of vapours of a substance = Molecular mass of that substance

**Average atomic mass and molecular mass**

A^{-} (Average atomic mass) = ΣA_{i} X_{i}/ΣX_{total }

M^{-} (Average molecular mass) = ΣM_{i} X_{i}/ΣX_{total }

Where A_{1} A_{2,} A_{3}…. are atomic mass of species 1, 2, 3,…. etc. with % ratio as X_{1 }X_{2} X_{3}…etc. Similar terms are for molecular masses.