Molecular mass of a substance also called as molecular weight and is obtained by multiplying the atomic mass of each element by the number of its atoms and then adding them together.

Molecular mass of methane (CH4) = (12.011 u)+4(1.008 u)

                                                             = 16.043 u

Gram molecular mass: The molecular mass of a substance expressed in grams is called its gram molecular mass.The amount of the substance is also called one gram molecules.

e.g Molecular mass of Caco3  = 100 u

      Gram molecular mass of Caco3 = 100

No of  g molecules or mole molecules =

Mass of substance in g

                                        = No of gram molecules x gram molecular mass

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