Chemistry: reactions, moles and pH
Chemistry questions in PECE are mostly from matter, atomic structure, chemical reactions, acids–bases–salts, metals, carbon compounds and periodic trends. Expect equation balancing, basic mole ideas and ‘which gas is evolved’ style practical facts.
Core rules
- Atomic structure: atomic number = protons; mass number = protons + neutrons; isotopes differ in neutrons.
- Moles: 1 mol = 6.022 × 10²³ particles = molar mass in grams. Moles = mass / molar mass.
- Reactions: balance atoms, not just charges; remember diatomic gases H₂, O₂, N₂, Cl₂.
- Acids and bases: pH < 7 acidic, 7 neutral, > 7 basic; litmus and phenolphthalein colour changes; neutralisation: acid + base → salt + water.
- Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au; a more reactive metal displaces a less reactive one from its salt solution.
Worked example
How many molecules are present in 22 g of CO₂? Reasoning: molar mass of CO₂ = 12 + (2 × 16) = 44 g/mol. Moles = 22/44 = 0.5 mol. Number of molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules. Note that the number of atoms is three times this, because each CO₂ molecule has three atoms.
Traps
Forgetting to multiply by two for O₂ or N₂; using atomic mass instead of molar mass; confusing pH 3 as twice as acidic as pH 6 (it is 1000 times more acidic); writing Pb as lead but then treating it as reactive as zinc; and forgetting that displacement needs a more reactive metal.