AP Chemistry 1.1 Moles and Molar Mass Study Notes - New Syllabus 2024-2025

Definition of a Mole

The mole is the SI unit for the amount of substance. It is defined as the amount of substance that contains as many particles (atoms, molecules, ions, or electrons) as there are atoms in exactly 12 g of carbon-12. This number is called Avogadro’s constant:

\( N_A = 6.022 \times 10^{23} \, \text{mol}^{-1} \) (Avogadro’s constant)

Key Relationships

1. Moles, mass, and molar mass:

\( n = \dfrac{m}{M} \)

where:

\( n \) = number of moles
\( m \) = mass of substance in grams
\( M \) = molar mass in g mol\(^{-1}\)

2. Moles and number of particles:

\( n = \dfrac{N}{N_A} \)

where:

\( N \) = number of particles
\( N_A = 6.022 \times 10^{23} \, \text{mol}^{-1} \)

Molar Mass

Molar mass is the mass of one mole of a substance, expressed in g mol\(^{-1}\). It is numerically equal to the relative molecular mass (Mr) or relative atomic mass (Ar), but with units.

Examples:

\( M(\text{H}_2\text{O}) = 2(1.01) + 16.00 = 18.02 \, \text{g mol}^{-1} \)
\( M(\text{NaCl}) = 22.99 + 35.45 = 58.44 \, \text{g mol}^{-1} \)

Applications of Moles and Molar Mass

1. Converting between mass and moles
2. Converting between particles and moles
3. Using balanced chemical equations to relate amounts of reactants and products