Chemistry: Quantitative Chemistry and Ionic Equilibrium
CG PET Chemistry tests Class 11–12 physical, inorganic and organic chemistry: mole concept, atomic structure and bonding, thermodynamics, equilibrium, electrochemistry, kinetics, p-block elements, hydrocarbons, biomolecules and polymers. Numericals are mostly mole- or equilibrium-based; theory questions target trends and exceptions.
Quantitative chemistry
- Mole: n = mass/molar mass = particles/6.022×10²³. One mole of an ideal gas occupies 22.4 L at STP (0 °C, 1 atm).
- Limiting reagent: the reactant giving the smaller product amount from the balanced equation controls yield.
- Molarity M = moles solute per litre of solution; dilution: M1V1 = M2V2 because moles do not change.
Equilibrium and pH
- At 25 °C, Kw = [H+][OH−] = 1.0×10−14; pH = −log[H+], pOH = −log[OH−], pH + pOH = 14.
- Strong acids/bases are fully ionized. For weak monobasic acid HA at concentration C, if Ka ≪ C, [H+] ≈ √(KaC); check ionization <5%.
- Le Chatelier: higher pressure favours fewer gas moles; a catalyst changes rate, not K.
Worked example
A 0.010 M solution of a weak monobasic acid HA has Ka = 1.0×10−5. Find pH. Let x = [H+]. Then x²/(0.010 − x) = 1.0×10−5. Assume x ≪ 0.010, so x² ≈ 1.0×10−7 and x ≈ 3.2×10−4 M. Ionization is 3.2%, so the assumption is acceptable. pH = −log(3.2×10−4) ≈ 3.49.
Traps
Applying 22.4 L mol−1 at temperatures other than 0 °C or pressures other than 1 atm; using solvent volume instead of solution volume for molarity; calculating pH of a weak acid as −log C; forgetting dilution; writing pH with too many decimal places (the number of decimal places should match the significant figures in [H+]).