Amount of substance

• Relative atomic mass (Ar) is the mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.
• Relative molecular mass (Mr) is the mean mass of a molecule of a compound, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.
• The mole is a unit of measurement for substances and always contains the same number of particles, known as the Avogadro Constant (L).
• The ideal gas equation relates pressure, volume, temperature, moles, and mass of a gas or volatile liquid under standard conditions.
• The empirical formula is the simplest whole number ratio of atoms of each element in a compound, while the molecular formula represents the true number of each atom in the molecule.