OCR Unit 1 - Atoms, bonds and groups


Measurement of relative masses

Instead of finding the mass of atoms directly, we compare the masses of different atoms , using the idea of relative mas. The carbon 12 isotope has been chosen as the international standard for the measurement of relative mass.

Relative isotopic mass

Relative isotopic mass: The mass of an atom of an isotope compared with one - twelfth of the mass of an atom of carbon-12.

Relative atomic mass, Ar

Relative atomic mass Ar: The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12

Contribution made by an isotope to the overall mass depends on:

  • Percentage abundance of the isotope.
  • The relative mass of the isotope.

Worked example: A sample of bromine contains 53.00% of bromine-79 and 47.00% of bromine-81. Determine the relative atomic mass of bromine.

Ar(Br) = (53.00/100 x 79.00) + (47.00/100 x 81.00) = 79.94

Relative molecular mass, Mr

Relative molecular mass, Mr: The weighted mean mass of a molecule compared with one twelfth of the mass of an atom of carbon-12.

Amount of substance and the mole

Amount of substance : The quantity whose unit is the mole. Chemists use "amount of substance" as a means of counting atoms.

The avagadro constant: The number of atoms per mole of the carbon-12 isotope (6.02 x 10^23 mol^-1)

Mole: The amount of any substance containing as many particles as there are carbon atoms in exactly 12g of the carbon-12 isotope.

Moles and gas volumes

Molar volume: The volume per mole of a gas. The units of molar volume are dm3 mol-1 . At room temperature and pressure, the molar volume is approximately 24.0 dm3 mol-1 .

Mols = mass/molar mass

Mols = Volume (dm3) / 24.0

Mols = Volume (cm3) / 24 000

Acids and Bases

Acid: A species that is a proton donor.

Base: A species that is a proton acceptor.


Salt: Any chemical compound formed from an acid when a H+ ion from the acid has been replaced by a metal ion or another positive ion.

Cation: A positively charged ion

Anion: A negatively charged ion.

Formation of Salts

acid + carbonate --> salt + carbon dioxide + water

acid + base --> salt + water

acid + alkali --> salt + water

acid + metal --> salt + hydrogen gas

Water of Crystallisation

Hydrated: Refers to a crystalline compound containing water molecules.

Anhydrous: Refers to a substance that contains no water molecules

Water of crystallisation: Refers to water molecules that form an essential part of the crystalline structure of a compound.

Working out the formula of a hydrated salt:

You can work out the formula of a hydrated salt using percentage or mass compositions. You will need to use the same method that you used to work out empirical forumla. However there is an additional step to show the water separately as its dot formula.

Determination of the formula of




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