KCET Chemistry - Mole Logic, Trends, and Distinguishing Tests
KCET Chemistry is 60 one-mark questions taken from the first and second PUC (NCERT Class 11–12) syllabus. Physical chemistry is numerical, inorganic chemistry is recall of trends and structures, and organic chemistry tests mechanism logic and distinguishing tests. With no negative marking and about 27 seconds per question, the efficient approach is to bank the short recall items and keep multi-step numericals for last.
Core rules to keep loaded
- Mole relations: n = m/M = V/22.4 L at STP for gases; molarity depends on temperature because volume does, molality does not.
- Equilibrium: Kp = Kc(RT)^Δn, and Le Chatelier's principle predicts the direction of shift only, not the new value.
- Organic reactivity: nucleophiles attack electron-poor carbon, electrophiles attack π clouds; carbocation stability governs bond cleavage.
- Hybridisation from steric number: 2 → sp, 3 → sp², 4 → sp³.
- Periodic trends: atomic radius falls across a period while ionisation enthalpy and electronegativity rise, with anomalies at half-filled and fully filled subshells.
Worked example
5.85 g of NaCl is dissolved to make 500 mL of solution. Molar mass = 23 + 35.5 = 58.5 g/mol, so n = 5.85/58.5 = 0.1 mol. Molarity = 0.1 mol ÷ 0.500 L = 0.200 M. Note the wording trap: adding the solid to 500 mL of water is not the same as making the volume up to 500 mL.
Traps that cost marks
Oxygen's oxidation number is −1 in peroxides and −2 elsewhere. Aryl halides resist nucleophilic substitution because the C–Cl bond gains partial double-bond character. Aldehydes reduce Tollens' reagent and Fehling's solution; ketones do not. Enantiomers are non-superimposable mirror images, not merely different conformers. When a reaction is reversed or halved, Kc must be inverted or square-rooted, never transplanted unchanged.