Amount of substance Revision Notes

Relative atomic mass – the weighted average mass of the atoms of an element, expressed in atomic mass units (amu) relative to –12C.\nRelative molecular mass – the sum of the relative atomic masses of all atoms in a molecule, expressed in amu relative to –12C.\nAvogadro constant – –6.022 ×10⁻23 particles per mole, the number of atoms, molecules, ions or formula units in one mole.\nMole concept – 1 mol = –6.022 ×10⁻23 entities.\nIdeal gas law – –pV = nRT.\n\n### Calculations\n- Molar mass (Mr) – add the relative atomic masses of all atoms in the formula.\n- Number of moles (n) – –n = m / Mr or –n = N / N_A.\n- Mass from moles – –m = n × Mr.\n- Concentration (c) – –c = n / V (L).\n- Ideal gas rearrangements – solve for p, V, n, R or T as required.\n- Empirical formula – determine the simplest whole‑number ratio from % composition or experimental mass data.\n- Molecular formula – multiply the empirical formula by an integer so that the calculated Mr matches the measured Mr.\n- Stoichiometry – use balanced equations to find reacting masses, limiting reagents, theoretical yield, actual yield, % yield and atom economy.\n- Titration – calculate unknown concentration from the mean titre, using the balanced equation and the molarity of the standard solution.