Chemistry › Definitions › Amount of substance

Amount of substance definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

47 terms

The mole, the Avogadro constant and molar mass

Mole: The amount of substance that contains 6.022 × 10²³ particles, the same number as there are atoms in 12 g of carbon-12.
Avogadro constant: The number of particles in one mole of a substance, 6.022 × 10²³ mol⁻¹, symbol L.
Amount of substance: The quantity that counts the particles in a sample, measured in moles, symbol n.
Relative molecular mass: The weighted mean mass of a molecule compared with one twelfth of the mass of an atom of carbon-12.
Relative formula mass: The weighted mean mass of a formula unit of a compound compared with one twelfth of the mass of an atom of carbon-12, used for ionic compounds.
Molar mass: The mass of one mole of a substance, in g mol⁻¹, numerically equal to its relative formula mass.
Water of crystallisation: Water molecules held in a fixed ratio within the crystal lattice of a hydrated salt.
Hydrated: Describes a crystalline compound that contains water of crystallisation.
Anhydrous: Describes a substance that contains no water of crystallisation.
Heating to constant mass: Repeating the cycle of heating, cooling and weighing until two successive masses agree, which shows the change is complete.

Ideal gas: A gas whose molecules have negligible volume and no intermolecular forces, so that it obeys pV = nRT exactly.
Absolute zero: The lowest possible temperature, 0 K or −273 °C, at which particles have the minimum possible energy.
Kelvin temperature: Temperature measured from absolute zero, in K, found by adding 273 to the temperature in °C.
Avogadro's law: Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.
Elastic collision: A collision in which the total kinetic energy of the colliding particles is unchanged.
Ideal gas equation: pV = nRT, relating the pressure, volume, amount and kelvin temperature of an ideal gas.
Gas constant: The constant R in the ideal gas equation, equal to 8.31 J K⁻¹ mol⁻¹.
Molar gas volume: The volume occupied by one mole of any gas at a stated temperature and pressure, 24.0 dm³ mol⁻¹ at room temperature and pressure.
Room temperature and pressure: About 293 K and 101 kPa, the conditions at which the molar gas volume is taken as 24.0 dm³ mol⁻¹.

Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
Molecular formula: The actual number of atoms of each element in one molecule of a compound.
Structural formula: The minimal detail that shows the arrangement of atoms in a molecule, group by group, such as CH₃CH₂OH.
Displayed formula: A formula showing the relative positioning of all the atoms in a molecule and every bond between them.
Structural isomers: Compounds with the same molecular formula but a different structural formula.
Empirical formula mass: The sum of the relative atomic masses of the atoms in the empirical formula.
Percentage by mass: The mass of an element in a compound as a percentage of the mass of the compound, equal to its mass in grams in 100 g.
Hydrocarbon: A compound that contains carbon and hydrogen only.
Combustion analysis: Finding the masses of carbon and hydrogen in a compound by burning a weighed sample in excess oxygen and weighing the carbon dioxide and water formed.

Concentration: The amount of solute, in moles, dissolved in each 1 dm³ of solution.
Mass concentration: The mass of solute dissolved in each 1 dm³ of solution, in g dm⁻³.
Standard solution: A solution whose concentration is known accurately.
Primary standard: A pure, stable, non-hygroscopic solid that can be weighed accurately to make a standard solution directly.
Titre: The volume of solution added from the burette to reach the end point.
End point: The point in a titration at which the indicator first changes colour permanently.
Equivalence point: The point at which the reactants have been added in exactly the amounts the equation requires.
Concordant titres: Titres that agree with each other to within 0.10 cm³.
Back titration: A method in which an excess of a reagent of known amount is added, and the excess left over is then found by titration.
Parts per million: The mass of solute in one million units of mass of solution, or for a gas the volume in one million units of volume of air.

Limiting reagent: The reactant that is completely used up first, and so determines the maximum amount of product.
In excess: Present in more than the amount needed to react with the limiting reagent, so that some is left at the end.
Theoretical yield: The mass of product that would form if all of the limiting reagent reacted as the equation shows.
Percentage yield: The actual mass of product obtained as a percentage of the theoretical maximum mass.
Actual yield: The mass of pure, dry product actually collected at the end of a preparation.
Overall yield: The yield of a synthesis of several steps, found by multiplying together the fractional yields of the steps.
Atom economy: The mass of the desired product as a percentage of the total mass of all the reactants, calculated from the balanced equation.
Addition reaction: A reaction in which two or more molecules join together to form a single product.
By-product: A substance formed in a reaction alongside the desired product.