Atomic and Molecular Mass

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Atomic and Molecular Mass

Atomic Mass : Atomic mass is the mass of a single atom of an element, expressed in atomie mass units (amu or u).

Standard Reference

Value

1amu  = 1.66 * 10 ^ - 27 kg

Examples of Atomic Masses

Hydrogen (H) = 11u

Oxygen (0) = 16u

Carbon (C) = 12u

Molecular Mass : Molecular mass is the sum of the atomic masses of all the atoms present in a molecule.

Formula : Molecular mass = 2 (Number of atoms Atomic mass of each element)

Unit : Same as atomic mass - atomic mass unit (amu or u)

Examples

Water (H₂O):

= (2 * 1) + (1 * 16) = 18u Carbon dioxide (CO₂):

= (1 * 12) + (2 * 16) = 44u Ammonia (NH2):

= (1 * 14) + (3 * 1) = 17u Methane (CH):

= (1 * 12) + (4 * 1) = 16u

Important Points to Remember

•     Atomic mass is used for individual elements


• Molecular moss is used for compounds/molecules


• Both are expressed in como/u


• Used in mole concept and stoichiometry