NSEC 2023 previous year paper · official paper and PYQ-pattern practice

NSEC 2023 PYQ-pattern questions

Showing 10 of the 10 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 · Classification of Elements and Periodicity

   Among the following ions, which one has the LARGEST ionic radius?

   • A. O²⁻
   • B. F⁻
   • C. Na⁺
   • D. Mg²⁺
   Answer and solution

   Answer: A

   All are isoelectronic (10 electrons), but O²⁻ has the smallest nuclear charge (Z=8), resulting in the weakest attraction on the electron cloud and thus the largest ionic radius. The radius decreases: O²⁻ > F⁻ > Na⁺ > Mg²⁺.

2. Q2 · Solutions

   A solution containing 3.0 g of a non-volatile solute in 100 g of water boils at 100.26°C at 1 atm. If Kb for water is 0.52 K·kg/mol, the molar mass of the solute is:

   • A. 30 g/mol
   • B. 45 g/mol
   • C. 60 g/mol
   • D. 75 g/mol
   Answer and solution

   Answer: C

   ΔTb = 0.26 K. Using ΔTb = Kb × molality: molality = 0.26/0.52 = 0.5 mol/kg. Moles of solute = 0.5 × 0.1 = 0.05 mol. Molar mass = 3.0/0.05 = 60 g/mol.

3. Q3 · Surface Chemistry

   Which of the following statements is correct regarding physical adsorption?

   • A. It is highly specific and involves formation of chemical bonds
   • B. It involves weak van der Waals forces and is reversible
   • C. It has high activation energy and is irreversible
   • D. It occurs only at high temperatures and is multi-layered always
   Answer and solution

   Answer: B

   Physical adsorption (physisorption) involves weak van der Waals forces between adsorbate and adsorbent, making it easily reversible and non-specific. It does not involve chemical bond formation and has low activation energy.

4. Q4 · General Principles of Isolation of Elements

   In the extraction of aluminium from bauxite ore, which of the following processes is used to purify the ore by removing iron oxide and silica impurities?

   • A. Froth flotation
   • B. Baeyer's process
   • C. Hall-Heroult process
   • D. Van Arkel method
   Answer and solution

   Answer: B

   Baeyer's process (also called Bayer process) is used to purify bauxite ore by treating it with concentrated NaOH solution. Aluminium oxide dissolves as sodium aluminate while iron oxide and silica remain as insoluble impurities and are filtered off.

5. Q5 · Polymers

   Which of the following polymers is formed by condensation polymerization?

   • A. Polystyrene
   • B. Neoprene
   • C. Dacron
   • D. Teflon
   Answer and solution

   Answer: C

   Dacron (polyethylene terephthalate) is formed by condensation polymerization between ethylene glycol and terephthalic acid with elimination of water molecules.

6. Q6 · Olympiad-style Problem Solving

   A compound X with molecular formula C₅H₁₀O₂ gives a positive iodoform test and does not react with Fehling's solution. Upon heating with concentrated H₂SO₄, it forms a cyclic compound. What is the most likely structure of X?

   • A. Pentanoic acid
   • B. 4-Hydroxypentanal
   • C. 4-Hydroxy-2-pentanone
   • D. Methyl butanoate
   Answer and solution

   Answer: C

   The iodoform test indicates a CH₃CO- group. No reaction with Fehling's means it's not an aldehyde. 4-Hydroxy-2-pentanone fits all criteria and can undergo intramolecular cyclization.

7. Q7 · Environmental Chemistry

   Which of the following statements about photochemical smog is correct?

   • A. It is primarily composed of sulfur dioxide and particulate matter
   • B. It forms mainly during cold winter mornings
   • C. Peroxyacetyl nitrate (PAN) is one of its key components
   • D. It results from incomplete combustion in reducing atmosphere
   Answer and solution

   Answer: C

   Photochemical smog contains PAN, ozone, and formaldehyde formed by photochemical reactions of NO₂ and hydrocarbons in sunlight, typically in warm, sunny conditions.

8. Q8 · Solutions

   A solution is prepared by dissolving 9.0 g of glucose (C₆H₁₂O₆, molar mass = 180 g/mol) in 200 g of water. What is the molality of the solution?

   • A. 0.15 m
   • B. 0.25 m
   • C. 0.45 m
   • D. 0.50 m
   Answer and solution

   Answer: B

   Molality (m) = moles of solute / kg of solvent. Moles of glucose = 9.0/180 = 0.05 mol. Mass of water = 0.200 kg. Molality = 0.05/0.200 = 0.25 m.

9. Q9 · General Principles of Isolation of Elements

   In the extraction of copper from copper pyrites, the process of roasting is primarily carried out to:

   • A. Remove arsenic impurities
   • B. Convert CuFeS₂ to Cu₂S and FeO
   • C. Obtain pure copper metal directly
   • D. Convert silica to slag
   Answer and solution

   Answer: B

   Roasting in controlled air converts the sulfides to oxides; CuFeS₂ is partially oxidized to Cu₂S and iron is oxidized to FeO, preparing for smelting.

10. Q10 · Aldehydes, Ketones and Carboxylic Acids

    Which of the following reagents can be used to distinguish between propanal and propanone?

    • A. Fehling's solution
    • B. 2,4-Dinitrophenylhydrazine
    • C. Sodium bisulfite
    • D. Lucas reagent
    Answer and solution

    Answer: A

    Fehling's solution oxidizes aldehydes (propanal) giving a red precipitate of Cu₂O but does not react with ketones (propanone), allowing clear distinction.