## 1.2 Amount of substance

When chemists measure out an amount of a substance, they use an amount in moles.

The mole is a useful quantity because one mole of a substance always contains the same number of entities of the substance. An amount in moles can be measured out by mass in grams, by volume in dm3 of a solution of known concentration and by volume in dm3 of a gas by using the ideal gas equation.

• Relative atomic mass and relative molecular mass - Definition  Relative atomic mass (Ar) in terms of 12C Definition    Relative molecular mass (Mr) in terms of 12C The term relative formula mass will be used for ionic compounds.
• The mole and the Avogadro constant - The Avogadro constant as the number of particles in a mole. The mole as applied to electrons, atoms, molecules, ions, formulas and equations. The concentration of a substance in solution, measured in mol dm-3. [Students will not be expected to recall the value of the Avogadro constant] Assessment and practical skills Students should be able to carry out calculations: • using the Avogadro constant
• The ideal gas equation - The ideal gas equation pV = nRT with the variables in SI units. [Students will not be expected to recall the value of the gas constant, R] Assessment and practical skills Students could be asked to find the Mr of a volatile liquid. Students understand that the correct units need to be in pV = nRT . Students carry out calculations with the ideal gas equation,
• Empirical and molecular formula - Empirical formula is the simplest whole number ratio of atoms of each element in a compound. Molecular formula is the actual number of atoms of each element in a compound. The relationship between empirical formula and molecular formula. Assessment and practical activities Students should be able to: • calculate empirical formula from data giving composition by mass or percentage by mass • calculate molecular
• Balanced equations and associated calculations - But, as total mass of products=total mass of reactants [Law of conservation of mass], then Assessment and practical opportunities • write balanced equations (full and ionic) for reactions studied • balance equations for unfamiliar reactions when reactants and products are specified. Students should be able to use balanced equations to determine: • masses • reacting volumes of gases • percentage yields •
• Mass spectrometry (AS and A2 analytical techniques) - This topic overlaps organic analysis for AS and A2. High resolution mass spectrometry can be used to determine the molecular formula of a compound from the accurate mass of the molecular ion. Understand that the fragmentation of a molecular ion: M+• → X+ + Y• gives rise to a characteristic relative abundance spectrum that may give information about the structure of