Chemistry: Mole Concept, Trends, Equilibrium, and Organic Basics
JENPAS-UG Chemistry tests mole-concept calculations, periodic trends, chemical bonding, equilibrium, and basic organic/biomolecule recognition. MCQs often combine a short calculation with a concept check.
Core rules and concepts
- Mole relations: n = m/M = N/N_A. At STP, one mole of an ideal gas occupies 22.4 L.
- Stoichiometry: balance the equation first; the limiting reagent determines the maximum product.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionization energy and electronegativity show the opposite pattern.
- Equilibrium: compare reaction quotient Q with K. If Q < K, the reaction shifts forward; if Q > K, it shifts backward. A catalyst changes only the rate of attainment, not the equilibrium position.
- Organic priority: COOH > CHO > C=O > OH > NH2 > ether when choosing the suffix and parent chain.
Worked example
How many grams of water form when 4 g H2 reacts with 32 g O2? The balanced equation is 2H2 + O2 → 2H2O. Moles H2 = 4/2 = 2 mol; moles O2 = 32/32 = 1 mol. The required ratio is 2:1, so both reactants are consumed exactly. Moles H2O = 2 mol, mass = 2 × 18 = 36 g. If either reagent were in excess, the limiting reagent would control the product yield.
Traps that cost marks
- Using mole ratios from an unbalanced equation.
- Applying 22.4 L mol⁻¹ to gases not at STP.
- Confusing atomic-radius trends with ionic-radius trends.
- Claiming a catalyst increases equilibrium yield.
- Choosing the wrong parent chain in IUPAC naming by ignoring functional-group priority.