Chemical Quantities and Reactions

Avogadro’s Number and the Mole

The mole is a fundamental unit in chemistry used to count small particles such as atoms, molecules, and ions. Avogadro’s number defines the number of particles in one mole.

• Avogadro’s Number: The number of particles (atoms, molecules, or ions) in one mole is .

• Equality: particles

• Conversion Factors:

• Historical Note: Avogadro’s number is named after Amedeo Avogadro (1776–1856), an Italian physicist.

Example: To find the number of molecules in 0.50 mole of CO2:

• CO2 molecules

Converting Between Moles and Particles

Avogadro’s number is used to convert between moles and the number of particles (atoms, molecules, or ions).

• To convert moles to particles: Multiply the number of moles by .

• To convert particles to moles: Divide the number of particles by .

Example: Number of atoms in 2.0 moles of Al:

• atoms of Al

Example: Number of moles in atoms of S:

• moles of S

Moles of Elements in a Formula

The subscripts in a chemical formula indicate the number of atoms of each element in one molecule and, equivalently, the number of moles of each element in one mole of the compound.

• Example: Aspirin, C9H8O4:

  • 1 molecule: 9 atoms C, 8 atoms H, 4 atoms O

  • 1 mole: 9 moles C, 8 moles H, 4 moles O

Example: Atoms of O in 0.150 mole of C9H8O4:

• atoms O

Molar Mass

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). For elements, it is numerically equal to the atomic mass in grams.

• Definition: Molar mass = mass of 1 mole of an element or compound (g/mol)

• Example: 1 mole of C atoms = 12.01 g of C atoms

Calculating Molar Mass for Compounds:

• Add the atomic masses of all atoms in the formula, multiplied by their subscripts.

• Example: Lithium carbonate, Li2CO3:

  • Li: 6.941 × 2 = 13.882

  • C: 12.01 × 1 = 12.01

  • O: 16.00 × 3 = 48.00

  • Total molar mass = 73.89 g/mol