The Mole and Avogadro’s Number - Chemistry

1. Fundamental Concepts

Mole (mol): One mole contains particles (atoms, molecules, ions, etc.).

Avogadro’s Number ((N_A)): , the number of particles in one mole of substance.

Molar Mass (M): The mass of one mole of a substance, in g/mol. It equals the relative atomic or molecular mass.

Relationship between amount of substance and number of particles:

where (n) = moles, (N) = number of particles, (N_A) = Avogadro’s number.

Relationship between moles and mass:

where (m) = mass in grams, (M) = molar mass in g/mol.

Combined usage: You can calculate number of particles, moles, or mass using these formulas.

Easy

1. Calculate the number of molecules in 2 mol of oxygen :

Medium

2. Calculate the number of atoms in 12 g of carbon :

Hard
3. Calculate the number of hydrogen atoms in 18 g of water :

Moles of water:
Each molecule has 2 H atoms:

1. Identify what is given: mass, moles, or number of particles.

2. Choose the right formula:

Moles → number of particles:

Mass → moles:

Number of particles → moles:

3. Check units: mass in grams, molar mass in g/mol.

4. Verify logic: particle numbers must be positive and reasonable.

5. Use chemical formula: multiply by the number of atoms in each molecule if needed.