NSEC 2025 previous year paper · official paper and PYQ-pattern practice

NSEC 2025 PYQ-pattern questions

Showing 9 of the 9 PYQ-pattern questions in this set — freshly worded in that year's pattern, not the original questions. Answers and solutions open under each question.

1. Q1 · Some Basic Concepts of Chemistry

   A sample of hydrated copper sulfate (CuSO₄·xH₂O) weighing 12.5 g was heated strongly until all water was driven off. The mass of the anhydrous residue was 8.0 g. What is the value of x?

   • A. 3
   • B. 5
   • C. 7
   • D. 10
   Answer and solution

   Answer: B

   Mass of water lost = 12.5 - 8.0 = 4.5 g. Moles of anhydrous CuSO₄ = 8.0/160 = 0.05 mol. Moles of H₂O = 4.5/18 = 0.25 mol. Ratio = 0.25/0.05 = 5, so x = 5.

2. Q2 · Chemical Thermodynamics

   For a reaction at 298 K, ΔH° = -92 kJ/mol and ΔS° = -200 J/(mol·K). The reaction will be spontaneous when:

   • A. At all temperatures
   • B. Only at temperatures below 460 K
   • C. Only at temperatures above 460 K
   • D. Never spontaneous
   Answer and solution

   Answer: B

   For spontaneity, ΔG° < 0. ΔG° = ΔH° - TΔS° = -92000 - T(-200) = -92000 + 200T. Setting ΔG° = 0: T = 92000/200 = 460 K. Since ΔH° is negative and ΔS° is negative, the reaction is spontaneous only at low temperatures (T < 460 K).

3. Q3 · d- and f-Block Elements

   The magnetic moment of a complex ion [Fe(CN)₆]³⁻ is measured to be approximately 1.73 BM. What is the number of unpaired electrons in this complex?

   • A. 0
   • B. 1
   • C. 2
   • D. 5
   Answer and solution

   Answer: B

   The magnetic moment μ = √(n(n+2)) BM, where n is the number of unpaired electrons. For μ = 1.73 BM, solving gives n = 1. [Fe(CN)₆]³⁻ contains Fe³⁺ (3d⁵) with CN⁻ as a strong field ligand causing pairing, leaving one unpaired electron in low-spin configuration.

4. Q4 · Organic Compounds Containing Nitrogen

   Which of the following amines will give a yellow oily compound on treatment with chloroform and alcoholic KOH (carbylamine test)?

   • A. N,N-Dimethylaniline
   • B. N-Methylaniline
   • C. Aniline
   • D. Trimethylamine
   Answer and solution

   Answer: C

   The carbylamine test is positive only for primary amines. Aniline is a primary aromatic amine and gives phenyl isocyanide (yellow oil) with CHCl₃ and alcoholic KOH.

5. Q5 · Chemistry in Everyday Life

   Ibuprofen is a commonly used analgesic and anti-inflammatory drug. It functions primarily by inhibiting which enzyme involved in prostaglandin synthesis?

   • A. Carbonic anhydrase
   • B. Cyclooxygenase (COX)
   • C. Acetylcholinesterase
   • D. Lactase
   Answer and solution

   Answer: B

   Ibuprofen inhibits cyclooxygenase (COX) enzymes, which are responsible for prostaglandin synthesis that mediates inflammation and pain.

6. Q6 · Environmental Chemistry

   Which international agreement aims to phase out the production and consumption of ozone-depleting substances like chlorofluorocarbons (CFCs)?

   • A. Kyoto Protocol
   • B. Paris Agreement
   • C. Montreal Protocol
   • D. Stockholm Convention
   Answer and solution

   Answer: C

   The Montreal Protocol (1987) is the international treaty designed to protect the ozone layer by phasing out production of ozone-depleting substances including CFCs.

7. Q7 · Surface Chemistry

   In the adsorption of a gas on a solid surface, which of the following statements is correct regarding physisorption?

   • A. It involves formation of chemical bonds between adsorbate and adsorbent
   • B. Enthalpy of adsorption is typically in the range of 200–400 kJ/mol
   • C. It is reversible and has low activation energy
   • D. It is highly specific and occurs only at active sites
   Answer and solution

   Answer: C

   Physisorption is characterized by weak van der Waals forces, making it reversible with low activation energy and enthalpy typically 20–40 kJ/mol, unlike chemisorption.

8. Q8 · d- and f-Block Elements

   Among the following transition metal ions in aqueous solution, which one exhibits the highest number of unpaired electrons?

   • A. Ti³⁺
   • B. V³⁺
   • C. Cr²⁺
   • D. Fe³⁺
   Answer and solution

   Answer: D

   Fe³⁺ has electronic configuration [Ar]3d⁵ with 5 unpaired electrons in high-spin octahedral complexes, the maximum among the given options.

9. Q9 · Haloalkanes and Haloarenes

   When 2-bromo-2-methylbutane is treated with potassium tert-butoxide in tert-butanol, the major product obtained is:

   • A. 2-Methylbut-1-ene
   • B. 2-Methylbut-2-ene
   • C. 3-Methylbut-1-ene
   • D. tert-Butyl 2-methylbutyl ether
   Answer and solution

   Answer: A

   Bulky base (t-BuOK) favors E2 elimination giving the less substituted (Hofmann) alkene, which is 2-methylbut-1-ene, over the more substituted Zaitsev product.