Key terms: The Mole Concept, Chemical Formula and Equation

Learn these definitions well enough to write them from memory, because Paper 2 regularly awards a mark for defining a term such as relative atomic mass, the mole, or the molar volume of a gas. Each entry gives the term and a precise definition; test yourself by covering the definitions and explaining each term aloud.

Relative mass terms

• Relative atomic mass (Ar), the average mass of one atom of an element compared with one-twelfth of the mass of one carbon-12 atom. It is a ratio and has no unit.
• Relative molecular mass (Mr), the sum of the relative atomic masses of all the atoms shown in the molecular formula of a substance. It also has no unit.
• Relative formula mass, the same total as relative molecular mass, but used for ionic compounds, which are made of ions rather than molecules; it is the sum of the relative atomic masses in the formula unit.
• Carbon-12 standard, the agreed reference for relative mass: exactly 12 units are assigned to one atom of the carbon-12 isotope, and all other relative masses are measured against one-twelfth of it.
• Relative isotopic mass, the mass of one atom of a particular isotope compared with one-twelfth of a carbon-12 atom; the average of these values for an element, weighted by abundance, gives the relative atomic mass, which is why some values, such as chlorine at 35.5, are not whole numbers.

The mole and particles

• Mole (mol), the amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. It is the SI unit for amount of substance.
• Avogadro constant, the number of particles in one mole, 6.02 x 10²³ mol⁻¹. One mole of any substance contains this many atoms, molecules or ions.
• Number of particles, the count of atoms, molecules or ions in a sample, found by multiplying the number of moles by the Avogadro constant.
• Molar mass (M), the mass of one mole of a substance, numerically equal to its relative atomic or molecular mass but expressed in grams per mole (g mol⁻¹).
• Amount of substance, the physical quantity that measures how much of a substance is present, expressed in moles; it is the quantity that the mole counts, distinct from mass and from volume.
• The mole equation, the central relationship number of moles = mass ÷ molar mass (n = m ÷ M), rearranged to find mass or molar mass as needed.

Gases

• Molar volume, the volume occupied by one mole of any gas under stated conditions. At room conditions it is 24 dm³ mol⁻¹, and at standard temperature and pressure (STP) it is 22.4 dm³ mol⁻¹.
• Room conditions, the everyday temperature and pressure at which the molar volume of a gas is taken as 24 dm³ mol⁻¹ in calculations.
• Standard temperature and pressure (STP), the reference temperature and pressure at which the molar volume of a gas is 22.4 dm³ mol⁻¹.
• Avogadro’s law, the principle that equal volumes of all gases, measured at the same temperature and pressure, contain the same number of molecules.

Formulae

• Chemical formula, a representation of a substance using element symbols and subscripts to show the number of each kind of atom, for example H₂O or CO₂.
• Empirical formula, the formula that shows the simplest whole-number ratio of atoms of each element in a compound, for example CH₂O.
• Molecular formula, the formula that shows the actual number of atoms of each element in one molecule; it is the empirical formula or a whole-number multiple of it, for example C₆H₁₂O₆.
• Ionic formula, the formula of an ionic compound, showing the simplest whole-number ratio of positive to negative ions that gives a neutral compound, for example NaCl or CaCl₂.
• Water of crystallisation, water molecules that form part of the structure of a crystalline salt, shown after a dot in the formula, as in CuSO₄·5H₂O; they must be included when calculating relative formula mass.
• Anhydrous salt, a salt that contains no water of crystallisation; heating a hydrated salt drives off the water to leave the anhydrous form, and the mass lost equals the mass of the water.
• Percentage composition by mass, the percentage of the total mass of a compound contributed by each element, found by dividing the mass of that element in the formula by the molar mass and multiplying by one hundred.

Equations and stoichiometry

• Reactant, a starting substance that reacts, written on the left-hand side of the arrow in a chemical equation.
• Product, a substance formed in the reaction, written on the right-hand side of the arrow in a chemical equation.
• Chemical equation, a representation of a reaction using formulae for the reactants and products, with an arrow from reactants to products and state symbols.
• Balanced equation, an equation in which the number of atoms of each element is equal on both sides, achieved by placing coefficients in front of formulae without changing any formula.
• State symbols, the labels (s) solid, (l) liquid, (g) gas and (aq) aqueous, written after each formula to show the physical state of that substance.
• Coefficient, a number written in front of a formula in a balanced equation to show how many moles of that substance take part.
• Mole ratio, the ratio of the numbers of moles of substances in a reaction, read directly from the coefficients of the balanced equation and used to convert between amounts.
• Stoichiometry, the study of the quantitative relationships, based on mole ratios, between the reactants and products in a chemical reaction.

Using the glossary

Do not just read these definitions; reproduce them. Cover the right-hand side and state each definition in a full sentence, then compare it with the wording here, paying attention to the parts that carry the mark, the comparison with carbon-12 for relative mass, the “simplest whole-number ratio” for empirical formula, and “equal number of molecules” for equal volumes of gas. A one-to-one teacher can check that your definitions are complete and precise, which is where easy Paper 2 marks are won or lost. Keep this glossary beside the revision notes and worked examples for this chapter, and return to it whenever a calculation question uses a term you are unsure of, so that the language of the mole concept becomes as automatic as the arithmetic across SPM Chemistry.