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Molecular Formula, Molecular Mass


 Molecular Formula

↪ Symbolic expression for a substance is called a formula.
↪ When the molecule of a substance is written with the help of symbols, we get what its called molecular formula.
↪ A molecular formula is the shorthand expression of the composition of a molecule. 
↪ It tells about the number and kinds of atoms which form a molecule.

⁕ Relative Molecular Mass

↪ It is defined as the mass of a molecule of substance with respect to the mass of 1/12th of an atom of C-12 isotope.
↪ Molecular mass of a substance is calculated by the summation of atomic mass of all atoms present in its molecular formulae.
eg: molecular mass of calcium carbonate (CaCO3) = 40+12+3x16 amu = 100 amu.

⁕ Significance of Molecular formula and % Composition

Like the symbols, the formula has a qualitative as well as quantative significance.
↪ the elements constituting the molecule e.g. HO tells that a molecule of water constitutes of hydrogen and oxygen.
↪ the number of atoms of each element present in one molecule of the given compound.
↪ the weight ratio and also the percentage of elements present in substance.
↪ the molecular weight of substance as a sum of atomic weights of all atoms present in it.
↪ the numerical coefficient written before the formula gives the number of molecules.

⁕ Empirical Formula
↪ It is the simplest formula of a compound that gives the ratio of atoms present in a molecule of the compound.
↪ It may not give the actual number of atoms present in a molecule but this formula is essential to establish molecular formula of a compound as 
Molecular formula = (Empirical Formula)n where n=1, 2, 3, etc.
↪ Empirical formula can be used to calculate % composition.