The Mole and Avogadro’s Constant

Full Notes:

What Is a Mole?

The mole (mol) is a counting unit in chemistry – like a "dozen", but much larger.

One mole is defined as exactly 6.02 × 10²³ elementary entities.

This huge number is called the Avogadro constant (N_A).

This allows chemists to count incredibly tiny particles – like atoms and molecules – in a practical way.

What Are Elementary Entities?

An elementary entity is any particle you’re counting in a chemical scenario. It could be:

• Atoms (e.g., one mole of He = 6.02 × 10²³ helium atoms)
• Molecules (e.g., one mole of H₂O = 6.02 × 10²³ water molecules)
• Ions (e.g., Na⁺, Cl⁻)
• Electrons
• Specified groups of atoms or particles

Always pay attention to what kind of entity the question is asking for!

Converting Between Moles and Particles

We can convert between the amount in moles and number of particles using:

Number of particles = n × N_A
n = Number of particles ÷ N_A

Where:

• n = number of moles
• N_A = 6.02 × 10²³ mol⁻¹
• Number of particles = atoms, molecules, ions, etc.

Summary

• The mole is the SI unit for the amount of substance.
• Avogadro’s constant is 6.02 × 10²³ mol⁻¹.
• A mole can represent atoms, molecules, ions, or electrons.
• Use N_A to convert between moles and number of particles.