Chemistry is the section where NEET candidates can gain the most in the least time, provided the three branches are revised as three separate habits. This set covers physical chemistry calculations, organic reactions and mechanisms, and inorganic chemistry including periodic properties, chemical bonding and coordination compounds. Organic questions show the mechanism step by step so the logic carries over to unfamiliar reactions.
For the reaction: A → B, if the order is 2 and rate constant k = 0.5 L·mol⁻¹·s⁻¹, find the half-life when initial concentration is 2 M.
Answer: A
For a second-order reaction, the half-life depends inversely on both the rate constant and the initial concentration.
Step 1: [Identify the Reaction Order and Given Data]
We are dealing with a second-order reaction (order = 2) with rate constant k = 0.5 L·mol⁻¹·s⁻¹ and initial concentration [A]₀ = 2 M. We need to find the time required for the concentration to reduce to half its initial value.
Order=2,k=0.5 L⋅mol−1⋅s−1,[A]0=2 M
Step 2: [Apply the Half-Life Formula for Second-Order Reaction]
For a second-order reaction, the half-life formula is derived from the integrated rate law and is expressed as:
t1/2=k[A]01
Substituting the values:
t1/2=0.5×21=1.01=1.0 s
The half-life is 1.0 s, so the answer is (A).
Q.25Medium
What is the entropy change when 18 g of ice at 273 K is converted to water at 273 K? (ΔfusH = 6 kJ/mol)
Answer: A
ΔS = ΔH/T = (6000 J/mol) / 273 K = 22.0 J/(mol·K). For 18 g (1 mole) of ice, ΔS = 22.0 J/K.
Q.26Medium
A buffer solution contains 0.1 M acetic acid and 0.1 M sodium acetate. If Ka = 1.8 × 10⁻⁵, calculate the pH:
The osmotic pressure of a solution is given by π = iMRT. If 0.1 M glucose solution has osmotic pressure 2.46 atm at 273 K, what is the van't Hoff factor?
Answer: B
i = π/(MRT) = 2.46/(0.1 × 0.0821 × 273) ≈ 1. Glucose is non-electrolyte with i = 1
Q.28Medium
For the exothermic reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), increasing temperature will:
Answer: B
For exothermic reactions, increasing temperature decreases Kc (unfavorable) and shifts equilibrium left (Le Chatelier's principle).
Q.29Medium
A buffer solution is prepared by mixing 100 mL of 0.1 M CH₃COOH and 100 mL of 0.1 M CH₃COONa. The pKa of acetic acid is 4.74. What is the pH?
For a spontaneous process at constant temperature and pressure:
Answer: C
For spontaneous process: ΔG < 0. Generally also ΔS > 0 for most spontaneous processes. (ΔH < 0 is not always required)
Q.34Medium
An endothermic reaction has ΔH = +50 kJ/mol. For this reaction to be spontaneous at all temperatures, which condition must be satisfied?
Answer: A
For an endothermic reaction (ΔH > 0) to be spontaneous, ΔG = ΔH - TΔS must be negative. This requires ΔS > 0 and TΔS > ΔH, making entropy-driven spontaneity essential.
Q.35Medium
The rate constant for a reaction doubles when temperature increases from 300 K to 310 K. What is the approximate activation energy (Ea)?
For the reaction: N₂O₄(g) ⇌ 2NO₂(g), if Kp = 0.5 atm at 298 K, what is Kc at the same temperature?
Answer: D
Relationship: Kp = Kc(RT)^Δn, where Δn = 2 - 1 = 1. Therefore, Kc = Kp/(RT) = 0.5/(RT). But using proper units, Kc = 0.5/(RT)² when pressure is in atm and volume in L.
Q.37Medium
What is the order of the reaction if its half-life is independent of initial concentration?
Answer: B
For first-order reactions, t₁/₂ = 0.693/k, which is independent of initial concentration. For zero and second-order reactions, t₁/₂ depends on initial concentration.
Q.38Medium
The cell potential (E°cell) for a reaction is -0.5 V. Which statement is correct?
Answer: B
When E°cell is negative, ΔG° = -nFE°cell is positive, making the reaction non-spontaneous. Also, negative E°cell indicates Kequilibrium < 1.
Q.39Medium
A solution containing 0.5 mol of a non-volatile, non-electrolyte solute in 1 L of water shows a freezing point of -0.93°C. What is the cryoscopic constant (Kf) of water? (Tf of pure water = 0°C)
Answer: B
ΔTf = Kf × m, where m = molality = 0.5 mol/1 kg (approximately). ΔTf = 0.93°C, so Kf = 0.093.5 = 1.86 K·kg/mol.
Q.40Medium
The osmotic pressure of a solution is 10 atm at 27°C. What is the molarity of the solution?
Answer: A
π = MRT, where π = 10 atm, T = 300 K, R = 0.0821 L·atm/(mol·K). M = π/(RT) = 10/(0.0821 × 300) ≈ 0.41 M.
