OCR CHEMISTRY A

Key Definitions

Amount of Substance (n) = measured in moles (mol), a quantity used by chemists to count atoms.

Molar Mass (gmol-1) = mass per mole of a substance.

Empirical Formula = simplest whole number ratio of atoms of each element in a compound.

Key Definitions

Isotope = an atom of the same element with a different number of neutrons and a different mass.

Relative Isotopic Mass = weighted mean mass of the isotopes of an element compared to 1/12 of Carbon-12

Relative Atomic Mass (Ar) = weighted mean mass of atoms of an element compared to 1/12 of Carbon-12

*Relative Molecular Mass (Mr) = e.g use for Cl2, CO2 (simple molecules) - mean mass of a molecule compared to 1/12 of Carbon-12

*Relative Formula Mass (Mr) = e.g use for NaCl (compounds with giant structures) - relative mass of an ion or an ionic compound compared to 1/12 of Carbon-12

*to calculate Mr you add together all the relative atomic masses (Ar) for all the atoms in a molecule/ion/ionic compound.

Key Notes: > The mass of an electron is 1/2000 and it has a negative charge.

> The mass of a proton is 1 and it has a positive charge.