Chemistry: Periodicity, Stoichiometry, Equilibrium and Organic Groups
Chemistry on KUUMAT covers physical, inorganic, and organic chemistry from the NEB 10+2 syllabus: atomic structure and periodicity, bonding, stoichiometry, equilibrium, thermodynamics, electrochemistry, and functional-group chemistry. The exam favours application-predicting a trend, balancing a redox change, or identifying a product-rather than recall alone.
Core rules
- Periodicity: across a period, effective nuclear charge rises, so atomic radius falls and ionisation energy generally rises; down a group, radius rises and ionisation energy falls.
- Mole concept: n = m/M = N/N_A; at STP one mole of ideal gas occupies 22.4 L. Use the limiting reagent to find theoretical yield.
- Equilibrium: Le Chatelier shifts oppose a change in concentration, pressure, or temperature; K changes with temperature only.
- Redox: balance atoms, then charge; oxidation number changes identify oxidant and reductant.
- Organic: classify by functional group; structural isomers differ in connectivity, stereoisomers in spatial arrangement.
Worked example
25.0 mL of 0.100 M HCl is diluted to 250.0 mL. Moles of HCl = 0.0250 L × 0.100 mol/L = 2.50×10⁻³ mol. New concentration = 2.50×10⁻³ mol / 0.250 L = 0.0100 M. HCl is strong, so [H⁺] = 0.0100 M and pH = −log(0.0100) = 2.00.
Traps
- Forgetting dilution changes concentration but not moles.
- Using initial moles for yield when a limiting reagent is present.
- Assuming a catalyst changes K; it changes only the rate.
- Missing chiral centres or assigning E/Z without priority rules.
- Letting significant figures drift: pH 2.00 is not pH 2.