# CH: Moles and molar mass # Moles and molar mass

Moles and molar mass are fundamental concepts in chemistry that help us quantify and relate the amounts of substances that are present.

### Moles

The mole is a unit of measurement used to count atoms, molecules, or formula units of a substance. It represents a specific amount of particles, similar to how a dozen represents 12 items. One mole (mol) is defined as the amount of substance that contains as many entities (atoms, molecules, etc.) as there are atoms in exactly 12 grams of carbon-12. This quantity is known as Avogadro’s Number, which is approximately 6.022 × 1023.

### Molar Mass

Molar mass refers to the mass of one mole of a substance. It is calculated by summing the atomic masses of all the atoms in a chemical formula. The atomic masses are taken from the periodic table and are expressed in atomic mass units (amu) or grams per mole (g/mol).

#### Example of molar mass calculation

The molar mass of water (H2O) can be calculated as follows:

Molar mass of H2O = (2 × atomic molar mass of hydrogen) + (1 × atomic molar mass of oxygen)

Molar mass of H2O = (2 x 1.01 g/mol) + (1 x 16.00 g/mol)

*The atomic molar masses of elements can be found on a periodic table.

Molar mass of H2O = 18.02 g/mol

Knowing the molar mass allows us to convert between the mass of a substance and the number of moles it contains. This is done using the following equation:

𝑴𝒐𝒍𝒆𝒔(𝒎𝒐𝒍)  = 𝑴𝒂𝒔𝒔 (𝒈) / 𝑴𝒐𝒍𝒂𝒓 𝑴𝒂𝒔𝒔 (𝒈/𝒎𝒐𝒍)

(Moles equals mass divided by molar mass)