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Molecular Mass

Molecular mass: Molecular mass of a molecule, of an element or a compound may be defined as a number which indicates how many times heavier is a molecule of that element or compound as compared with 1/12 of the mass of an atom of carbon–12. Molecular mass is a ratio and hence has no units. It is expressed in a.m.u.

Molecular mass = mass of one molecule of the substance/1/12×Mass of one atom of C-12       
Actual mass of one molecule = Mol. mass  ×1.66×1024 gm

Molecular mass of substances is the additive property and can be calculated by adding the atomic masses present in one molecule.


Gram molecular mass (GMM) and Gram molar volume :
Molecular mass of an element or compound when expressed in gm. is called its gram molecular mass, gram molecule or mole molecule.


Number of gm molecules = Mass of substances/GMM

Mass of substances in gm = No. of gm. molecules × GMM
Volume occupied by one mole of any gas at STP is called Gram molar volume. The value of gram molar volume is 22.4 litres. Volume of 1 mole of any gas at STP = 22.4 litres    

Expression for mass and density

Mass of  of any gas at STP = V.D. of that gas in gm.
Density of a gas at NTP =  Mol. Mass in gm./22400ml

Important generalisations

Number of atoms in a substance
= Number of GMM 6.02 × 1023 × Atomicity 
Number of electrons in given substance

= Number of GMM × 6.02 × 1023 × Number of electrons

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