Molecular weight And Mole

- “The molecular weight of substance is the relative mass of 1 molecule of it compared with 1/12 of the mass of an atom of carbon 12 isotope.” Molecular weight can be calculated by summing up the atomic weights of its constituent atoms. Eg. molecular weight of H₂SO₄ is 2 × 1 + 1 × 32 + 4 × 16 = 98

- According to Berzelius Hypothesis, “Equal volumes of all gases under the same conditions of temperature and pressure contain same number of atoms.”

- According to Avogadro’s Hypothesis, “Equal volume of all gases under the same conditions of temperature and pressure contain same no. of molecules.”

- Avogadro’s hypothesis leads us to following Important deductions.

1. Atomicity of elimentary gases.

2. Relationship between molecular weight and vapour density i.e. molecular weight of a gas is twice, it’s vapour density.

3. Gram molecular volume of gases i.e. 1 gram mole (molecular mass expressed in gram) of all gases occupies 22.4 litres at NTP. For example, 1 mole (32 gram) of oxygen or 1 mole (2 gram) of hydrogen at NTP occupies 22.4 litres.

4. 1 mole of any substance contains equal number of molecules called Avogadro’s number and is equal to 6.023 × 10²³.

5. Determination of molecular formula from volumetric composition.

6. Mole Concept : Molecular mass expressed in terms of gram is called gram molecular mass or in short mole. The mole of substance can be calculated as,

No of Mole =

Mass in gram

Molecular wt.

E.g. Calculate the no. of mole in 80 gm of oxygen.

Solution: No of Mole =

Mass in gram

Molecular wt.

= 80/32 = 2.5

Determination of molecular weight by victor Meyer’s method :

We know that,

Molecular weight = 2 × vapour density and

Vapour density =

weight V_cc of substance at NTP

Weight V_cc of hydrogen at NTP