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Definition of molecular mass : the mass of a molecule that is equal to the sum of the masses of all the atoms contained in the molecule

First Known Use of molecular mass : 1873

it is measured in daltons (Da or u)

The molecular mass and relative molecular mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g/mol. The molar mass is usually the more appropriate figure when dealing with macroscopic (weigh-able) quantities of a substance.



Molecular masses are calculated from the atomic masses of each nuclide present in the molecule, while molar masses are calculated from the standard atomic weightsof each element. The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be “normal”).

For example,

i) Relative molecular mass of hydrogen molecule (H2)


= 2 × (relative atomic mass of hydrogen atom)


= 2 × 1.008 u


= 2.016 u.




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