Amount of substance (moles): key terms
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Amount of substance
The quantity whose unit is the mole, used to count particles.
Examiners say Write "amount of NaCl", not "amount of salt": OCR uses the formula of the substance.
Mole
The amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12: 6.02 × 10²³ particles.
Examiners say Say what the particles are: atoms, molecules, ions or electrons.
Avogadro constant
The number of particles per mole, 6.02 × 10²³ mol⁻¹.
Examiners say It is on the Data Sheet. Multiply moles by it to get particles; remember to multiply again if the question asks for atoms or ions in each formula unit.
Molar mass
The mass per mole of a substance, in g mol⁻¹.
Examiners say n = m ÷ M. Use the relative atomic masses from the Periodic Table as printed.
Molar gas volume
The volume per mole of a gas, in dm³ mol⁻¹: 24.0 dm³ mol⁻¹ at room temperature and pressure.
Examiners say Only use 24.0 at RTP. At other conditions use pV = nRT with Pa, m³ and K.
Empirical formula
The simplest whole-number ratio of atoms of each element present in a compound.
Examiners say Divide each mass or percentage by the relative atomic mass, then by the smallest answer. Do not round 1.5 or 1.33 to a whole number: multiply up.
Molecular formula
The number and type of atoms of each element in a molecule.
Examiners say Molecular formula = empirical formula × (Mr ÷ empirical formula mass).
Water of crystallisation
Water molecules that are part of the crystalline structure of a hydrated salt.
Examiners say Anhydrous means with no water of crystallisation. Heat to constant mass to be sure all the water has gone.
Percentage yield
Actual amount of product ÷ theoretical amount of product × 100.
Examiners say Work in moles, and find the limiting reagent first.
Atom economy
Sum of the molar masses of the desired products ÷ sum of the molar masses of all products × 100.
Examiners say Use the balanced equation, including the balancing numbers. Addition reactions have 100% atom economy.
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