Amount of substance study guide

1. Relative Masses\n- Relative atomic mass (Ar) – weighted average of the masses of the naturally occurring isotopes of an element, expressed in amu relative to –12C.\n- Relative molecular mass (Mr) – sum of the Ar values of all atoms in a molecule.\n- Relative formula mass for ionic compounds – sum of the Ar values of the cation and anion, multiplied by the stoichiometric coefficients.\n\n# 2. The Avogadro Constant and the Mole\n- Avogadro constant (N_A) – –6.022 ×10⁻23 particles per mole.\n- Mole (n) – the amount of substance that contains N_A entities.\n- Conversions –\n - n = m / Mr\n - n = N / N_A\n - m = n × Mr\n - N = n × N_A\n\n# 3. Ideal Gas Law\n- Equation – pV = nRT\n- Rearrangements – solve for any variable.\n- Units – p in Pa, V in m³, n in mol, R = 8.314 J mol⁻¹ K⁻¹, T in K.\n- Unit consistency – essential for correct results.\n\n# 4. Empirical and Molecular Formulae\n- Empirical formula – simplest whole‑number ratio of elements.\n - From % composition: divide each % by the atomic weight, then divide by the smallest ratio.\n - From experimental mass: convert masses to moles, divide by the smallest mole value, round to nearest whole number.\n- Molecular formula – empirical formula × integer factor such that calculated Mr matches the measured Mr.\n- Combustion analysis – use CO₂ and H₂O masses to find C and H, then deduce other elements.\n\n# 5. Stoichiometry and Yields\n- Balanced equations – required for all calculations.\n- Limiting reagent – the reactant that produces the least amount of product.\n- Theoretical yield – maximum product obtainable from the limiting reagent.\n- Actual yield – experimentally obtained product.\n- Percentage yield – (actual / theoretical) × 100%.\n- Atom economy – (molecular weight of desired product / sum of molecular weights of all reactants) × 100%.\n- Yield vs. atom economy – yield measures how much product is obtained; atom economy measures how efficiently atoms are used in the reaction.\n\n# 6. Titration\n- Standard solution – prepare by dissolving a known mass of a primary standard in a known volume of solvent.\n- Mean titre – average of several titre readings.\n- Calculating concentration – use the balanced equation and the molarity of the titrant to find the unknown concentration.\n\n# 7. Practical Skills\n- Volumetric solution preparation – accurate weighing, dissolution, transfer to a volumetric flask, and final volume adjustment.\n- Acid–base titration – use a suitable indicator, record the volume of titrant at the endpoint, and calculate the concentration of the unknown solution.\n\n# 8. Practice Questions (see Practice Test section)