Chemistry
What the Chemistry section tests
CUSAT CAT chemistry mixes numerical physical chemistry with factual inorganic and mechanism-based organic chemistry. Questions often ask for a single decisive fact: a periodic trend, a coordination number, a major product, or a pH.
Core rules to keep ready
- Mole concept: n = m/M = N/N_A; at STP one mole of ideal gas occupies 22.4 L. For limiting-reagent problems, convert every reactant to moles and compare with stoichiometric coefficients.
- Weak-acid pH: [H⁺] ≈ √(K_a·C) when C/K_a > 100; otherwise solve the quadratic. pH = −log[H⁺].
- Le Chatelier: a system at equilibrium shifts to oppose the imposed change; adding an inert gas at constant volume does not shift a gas-phase equilibrium.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionisation energy and electronegativity follow the reverse pattern, with exceptions at half-filled and fully filled subshells.
- VSEPR: count bond pairs plus lone pairs; lone-pair repulsion is stronger than bond-pair repulsion, compressing bond angles (CH₄ 109.5°, NH₃ 107°, H₂O 104.5°).
- Organic: Markovnikov addition to alkenes; SN1 is favoured by tertiary substrates and polar protic solvents, SN2 by primary substrates and strong nucleophiles.
Worked example
Find the pH of 0.01 M acetic acid (K_a = 1.8 × 10⁻⁵). Check the approximation: C/K_a = 0.01/(1.8 × 10⁻⁵) ≈ 556 > 100, so [H⁺] ≈ √(K_a C) = √(1.8 × 10⁻⁷) = 4.24 × 10⁻⁴ M. pH = −log(4.24 × 10⁻⁴) = 4 − log 4.24 ≈ 4 − 0.63 = 3.37. Because [H⁺] is about 4% of C, the approximation is acceptable.
Traps that cost marks
- Using 22.4 L at any temperature and pressure, not only at STP.
- Forgetting that pH is logarithmic, so a tenfold dilution changes pH by one unit, not by ten.
- Assigning the wrong oxidation state in coordination compounds or ignoring the charge on the complex.
- Drawing the minor product when the question asks for the major Markovnikov or anti-Markovnikov product.