Question

what do you mean by valency, molecular mass

Answer 1

the combining capacity of an element is called valency.
the mass of molecule is called molecular mass.

Answer 2

the valence of an elementis a measure of its combining power with other atoms when it forms  moleculesThe concept of valence was developed in the second half of the 19th century and was successful in explaining the molecular structure of inorganic and organic compounds
Molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the atomic weights of each constituent element multiplied by the number of atoms of that element in the molecular formula.
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