OCR AS Level Chemistry A Past Papers

Mole calculation questions follow a reliable sequence: convert the given quantity to moles (using moles = mass/molar mass, or moles = concentration × volume in litres, or moles = volume of gas at STP / 24.0 dm³), use the stoichiometric ratios from the balanced equation to find moles of the target substance, then convert back to the required unit (mass, volume, or concentration). Students lose marks by omitting units, using incorrect molar masses, or failing to account for the stoichiometric ratio. Write out each conversion step explicitly. For bonding questions, the hierarchy of bond strength must be memorised: ionic bonds and covalent bonds are strong intramolecular forces (bonds within a molecule); van der Waals forces, permanent dipole-dipole interactions, and hydrogen bonds are weaker intermolecular forces (forces between molecules). Melting and boiling point questions assess intermolecular forces — the stronger the intermolecular forces, the more energy is needed to separate the molecules, and therefore the higher the melting or boiling point. Hydrogen bonds are the strongest intermolecular forces, which is why water has an anomalously high boiling point relative to its molar mass. For organic mechanism questions, nucleophilic substitution in halogenoalkanes requires you to draw curly arrows correctly. In SN2 (primary halogenoalkanes): the nucleophile attacks the carbon from the back (180° to the leaving group), the C–X bond breaks simultaneously, and the configuration at the carbon inverts (Walden inversion). In SN1 (tertiary halogenoalkanes): the C–X bond breaks first to form a stable carbocation intermediate, then the nucleophile attacks. Draw both mechanisms with correct curly arrows, clearly showing the origin and destination of each electron pair.