PSEB SS Chemistry cheat sheet

Mole concept: n = m/M = N/N_A = V/22.4 dm³ at STP; molar volume of an ideal gas is 22.4 dm³ mol⁻¹ at 273 K and 1 atm (22.7 dm³ at 100 kPa).

Ideal gas equation PV = nRT with R = 8.314 J K⁻¹ mol⁻¹; for gas mixtures, partial pressure P_i = x_i × P_total (Dalton's law).

First law: ΔU = q + w; work done on the system w = −PΔV. Enthalpy change ΔH = ΔU + Δ(PV); ΔH is negative for exothermic reactions.

Hess's law: ΔH for a reaction is path-independent, so ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants).

Equilibrium constant expressions use activities of products over reactants, each raised to its stoichiometric coefficient; pure solids and liquids are omitted. K_p = K_c(RT)^Δn.

Le Chatelier: increasing pressure shifts equilibrium toward the side with fewer gas moles; raising temperature favours the endothermic direction; a catalyst changes rate but not K.

pH = −log[H⁺]; for a weak acid pH = ½(pK_a − log c). Buffer: pH = pK_a + log([salt]/[acid]) (Henderson–Hasselbalch).

Electrochemistry: E°_cell = E°_cathode − E°_anode; ΔG° = −nFE°_cell; at 298 K, E°_cell = (0.059 V/n) × log K.

Quantum model: orbitals described by n, l, m_l, m_s; max electrons in a shell = 2n²; Hund's rule and Pauli exclusion govern ground-state configurations.

Periodic trends: atomic radius decreases across a period and increases down a group; ionisation energy and electronegativity increase across a period and decrease down a group.

Bonding: VSEPR gives shape from electron-pair repulsion (lone pair > lone pair > bond pair); sp, sp², sp³ hybrids correspond to 180°, 120°, 109.5°.

Organic: functional-group priority for IUPAC naming is acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > halide.

Reaction kinetics: rate = k[A]^m[B]^n; first-order half-life t½ = 0.693/k and is independent of initial concentration.

Titration calculation: moles of analyte = M × V(L) × stoichiometric ratio; at the end point, moles of acid and base relate through the balanced equation, not 1:1 by default.

Standard solution preparation: weigh on an analytical balance, dissolve, transfer quantitatively with washings to a volumetric flask, and make up to the mark at eye level (meniscus bottom).

Primary standards (e.g. oxalic acid, KHP, Na₂CO₃, AgNO₃) are pure, stable, non-hygroscopic and have high molar mass; NaOH and KMnO₄ are not primary standards.

Burette readings are recorded to 0.05 cm³ (two decimal places); the titre is the difference between final and initial readings, and concordant titres should agree within 0.10 cm³.

KMnO₄ titrations are self-indicating: the end point is a persistent pale pink; the medium is acidified with dilute H₂SO₄, never HCl (chloride is oxidised).

Iodometric titration: starch indicator is added near the end point when the yellow iodine colour fades to pale straw; the blue-black starch–iodine complex marks the end point.

Flame tests: Li crimson, Na golden yellow, K lilac (viewed through cobalt glass), Ca brick red, Sr crimson, Ba apple green, Cu blue-green.

Qualitative cation tests: NH₄⁺ gives ammonia on warming with NaOH; Pb²⁺ gives a white precipitate with dilute HCl soluble on heating; Cu²⁺ gives a pale blue precipitate with NaOH.

Qualitative anion tests: CO₃²⁻ effervesces with dilute acid (CO₂ turns limewater milky); SO₄²⁻ gives a white precipitate with BaCl₂ insoluble in dilute HCl; Cl⁻ gives a white precipitate with AgNO₃ soluble in ammonia.

Gas tests: H₂ burns with a pop; O₂ relights a glowing splint; CO₂ turns limewater milky; Cl₂ bleaches moist litmus; SO₂ decolourises acidified KMnO₄.

Melting-point determination: a pure solid melts sharply over 1–2 °C; a depressed or widened range indicates impurity. Impure solids also lower and broaden the melting range.

Chromatography: R_f = distance travelled by solute / distance travelled by solvent front; R_f is less than 1 and depends on solvent, temperature and stationary phase.

Gravimetric analysis: digest the precipitate, filter, wash, dry or ignite to constant mass, then calculate the mass of the sought component from the weighed form using mole ratios.

Safety rule: never pipette by mouth, add concentrated acid to water (not water to acid), and heat test tubes pointing away from people; report observations, not conclusions, in the record.