Mole, Mass and Number Relationship

1 mol  = 6.022 × 10²³ elementary entities

Molar mass  = Mass of 1 mole of substance = Mass of 6.022 × 10²³ elementary entities

Atomic Mass Unit

By international agreement, a unit of mass to specify the atomic and molecular masses has been defined. This unit is called atomic mass unit and its symbol is amu. The mass of one C-12 atom, is taken as exactly 12 amu. Thus, C-12 atom serves as the standard.

The Atomic mass unit is defined as a mass exactly equal to the 1/12^th of the mass of one carbon-12 atom.

Relative Atomic and Molecular Masses

Relative masses of atoms and molecules are the number of times each atom or molecules is heavier than 1/12^th of the mass of one C-12 atom.

Example: Relative molecular mass of water, H₂O

H₂O = (2 × relative atomic mass of H) + (relative atomic mass of O)

= (2 × 1) + (16) = 2 + 16 = 18

The relative atomic and molecular masses are just numbers and dimensionless, unit-less quantities.

Molar Mass

Molar mass is the mass of 1 mol of the substance.

Molar mass of an element

The relative atomic mass of carbon–12 is 12. A 12 g sample of it would contain 6.022 × 10²³ atoms. Hence, the molar mass of C-12 is 12 g mol^–1.

Number of Moles

Molar mass of water is 18 g mol^–1. If you have 18 g of water, you have 1 mol of it. Suppose you have 36 g water (18 × 2), you have 2 mol of it.