chap 3 Atoms, molecules and stoichiometry

Unified atomic mass unit

Onw twelfth of the mass of a carbon-12 atom

Relative isotopic mass

The mass of a particular atom of an isotope compared to the value of the unified atomic mass unit

Formula unit

The simplest formula for a covalent giant structure or an ionic giant structure. E.g. MgCl2

Relative molecular mass, Mr

The weighted average of a mass of molecule in a given sample of that molecule compared to the value of the unified atomic mass unit

Relative formula mass, Mr

the weighted average mass of one formula unit compared to the value of the unified atomic mass unit

Relative atomic mass, Ar

the weighted average mass of atoms in a given sample of an element compared to the value of the unified atomic mass unit

Water of crystallisation

A specific number of moles of water associated with a crystal structure

Hydrated compound

compound whihc contains a definite number of moles of water in their structure

anhydrous

containing no water of crystallisation

relative atomic abundance

the proportion of one particular isotopes in a mixture of isotopes, usually expressed as a percentage. The heights of the peaks in a mass spectrum show the proportion of each isotope present

Molecular ion

the ion that is formed by the loss of an electron from the original completed molecule during mass spectrometry. This gives us the mass of an unknown compound

Fragmentation

the breaking up of a covalent compound during mass spectrometry into smaller positively charged species.

Molecular formula

the formula that shows the number and type of each atom in a molecule

E.g. molecular formula of ethanol is C2H6O

Mole

the amount of substance which contains 6.02 x 10^23 specified particles (atoms, molecules, ions or electrons)

Avogadro constant, L

the number of specified particles (atoms, ions, molecules or electrons) in a mole of those particles. Its numerical value is 6.02 x 10^23

Molar mass

the mass of a mole of a substance in grams

Number of mol= mass of substance in grams (g) / molar mass (g/mol)

Stoichiometry

the mole ratios of reactants and products shown in the balanced equation.

Emperical formula

the simplest whole number ratio of the elements present in one molecule or formula unit of the compound

Oxidation number (oxidation state)

A number given to an atom or ion in a compound that describes how oxidised or reduced it is.

Compound ion

An ion containing more than one type of element. E.g OH-

Spectator ions

ions present in a reaction mixture which do not take part in the reaction

IOnic equations

a balanced equation showing only those ions, atoms or molecules taking part in the reaction. Spectator ions are not shown. Ionic reactions are often written for reactions involving a change in oxidation state.

Solute

a substance which dissolves in a solvent to form a solution

Solvent

a substance which dissolves the solute to form a solution

Concentration

Concentration (mol/dm^3) = number of moles of solute (mol) / volume of solution (dm^3)