Molar mass

Molar mass is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance. The base SI unit for molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed in g/mol.

Definition

In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in moles. For a pure substance, the molar mass is the same as the atomic weight of the element. For a mixture, it is the sum of the molar masses of its constituent elements, each multiplied by the number of times it appears in the chemical formula.

Calculation

The molar mass of a substance can be calculated by summing the molar masses of all the atoms in its chemical formula. The molar mass of an element is found on the periodic table, and it is the element's atomic weight in g/mol. For example, the molar mass of oxygen is approximately 16 g/mol.

Applications

Molar mass is used in stoichiometric calculations in chemistry to relate amounts of a substance to the chemical equation that describes its formation or reactions. It is also used to convert between the mass of a substance and the amount of a substance.

See also

• Atomic mass
• Mole (unit)
• Stoichiometry

Molar mass gallery

• Mass versus moles of iron vs gold
  Mass versus moles of iron vs gold