2.1.3/2.1.4/2.1.5 Amount of substance, acids and redox GapFill

Amount of substance, n, is measured in   molesparts per millionwave numberspascals. One of these contains 6.02 × 10²³ (a value known as   Lavoisier'sAvogadro'sLe Chatelier'sHaber's constant) particles, be they atoms, molecules, electrons or formula units. For a gas, if the pressure, temperature and volume of the container are known, the amount of substance can be estimated using the   idealabstracttheoreticalabsolute gas equation.

Chemical compounds can be represented by two types of formula: one that gives the simplest whole-number ratio of the number of atoms of each element is the   generaldisplayedempiricalmolecular formula, while one that gives the actual number of atoms of each element is the  moleculardisplayedempiricalgeneral formula.

In a chemical reaction, the relative amount of each reactant and product, as represented by the coefficients in the reaction equation, is called the stoichiometry. Once understood, it can be used to calculate many useful quantities, including the actual amount of product produced as a percentage of the theoretical maximum (percentage   returnrealisationyieldacquisition), and how much of the reactants are incorporated into the desired starting material (atom  economydecayefficiencyutility).

When dissolved in water, acids release  NH4+OH−H+Cl− ions, with those that fully dissociate said to be '  stronghardsuperheavy acids'. The reaction of these ions with OH⁻ ions is a form of  negationannulmentannihilationneutralisation reaction. The concentration of an acidic (or basic) solution can be determined using   chromatographytitrationcalorimetryelution.

Redox reactions involve the transfer of   electronsneutronsprotonsions between species, and a change in their oxidation numbers (also known as oxidation   classesstagesgradesstates). The species that is oxidised loses electrons and experiences an increase in oxidation number; the species that is   reducedminimiseddiminishedlessened gains electrons and experiences a decrease in oxidation number. In the name of a compound, the oxidation number of an element is expressed as a Roman numeral; for +1, +2 and +3, the numerals will be  X, XX and XXXI, II and IIIV, VV and VVVL, LL and LLL, respectively.