Amount of substance: symbol n, a quantity fundamental to chemistry. Atoms and molecules are much too small or light to be counted or weighed individually in the laboratory. The chemist therefore needs a unit to specify the quantity amount of substance of an appropriate magnitude (size) for laboratory or industrial scale work. The chosen unit is the mole.
Mole: symbol mol, the unit of the quantity amount of substance of a system, the amount of substance which contains as many elementary entities as there are atoms in 12 grams of carbon-12 (i.e. carbon consisting only of the isotope 12C).
12 g is an easily measurable mass. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
It follows from this definition of the mole that x moles of dihydrogen (H2,) will contain exactly the same number of dihydrogen molecules as there are dioxygen molecules (O2) in x moles of dioxygen or water molecules (H2O) in x moles of water. Thus it follows from the chemical equation
Molar mass: symbol M, the mass per mole of substance (the substance being defined by its chemical formula). (Molar means per mole in this context.)
The amount of substance is normally measured by weighing, (i.e. by measuring its mass using a balance), and the amount in moles determined by rearranging this equation to give
For a liquid the volume may be measured and this converted to amount in moles by using both the mass and density of the liquid.
Stoichiometry: The quantitative relationship between the amounts of reactants consumed and products formed in a chemical reaction as expressed by its balanced chemical equation. The general chemical equation
The most useful expression for the stoichiometry of the above general chemical equation is
This equation and are two of the most important equations used in practical quantitative chemistry.
Also sometimes called unified atomic mass unit, symbol u, previously amu.
Relative atomic mass: Symbol Ar, mean mass of one atom of an element (i.e. taking into account the relative natural abundance of the isotopes) relative to (i.e. divided by) mm. With the exception of the heaviest elements which have been formed from different radioactive isotopes, in general the relative amounts of the different isotopes of an element is independent of its source.
Note that the relative atomic mass has no units because it is the ratio of two masses. Most reference books and periodic tables simply give Ar values for the elements. From a practical viewpoint it is most important to realise that the numerical value of the molar mass M(E) is equal to Ar(E) when the units of molar mass are g mol−1 .
Relative molecular mass: Symbol Mr, the mass of one molecule of the substance relative to mμ. It is simply the sum of all the Ar values of the atoms in the molecule.
Molecular weight: An older term for molar mass, but sometimes used for relative molecular mass.
Determine the amount of substance in the given masses of the following compounds.
3. 75 kg of C6H6
Determine the masses of the given amounts of the following compounds.