Plus One Chemistry Annual Exam Answer Key March 2026

📘 CHEMISTRY  —  ANSWER KEY

Part III · Time 2 hrs Max. 60 scores Cool-off 15 min

✦ Questions 1 – 5 (1 point each, any four)

1. Find the number of significant figures in 0.04597 g.

4 (leading zeros not significant)

2. Pauling scale – element with highest electronegativity

Fluorine (F)

3. Identify the incorrect Lewis structure.

(b) N=O – incorrect valence / odd electron issue.

4. Equilibrium constant K_c depends on:

(a) temperature

5. Structural formula of But-2-en-1-ol

CH₃–CH=CH–CH₂–OH

✦ Questions 6 – 15 (2 scores each)

6. 96 g CH₄ combustion → mass of CO₂ ?

264 g (6 mol CH₄ → 6 mol CO₂ = 264 g)

7. Radius of third orbit of H (a₀ = 52.9 pm)

476.1 pm (r = n²·a₀ = 9×52.9)

8. (i) Correct electronic config of Carbon

1s² 2s² 2p²
(ii) Sketch of p orbital: two lobes with nodal plane.

(a drawing should show two lobes on opposite sides of nucleus)

9. Two types of hydrogen bonding + example

Intermolecular (H₂O···H₂O) & Intramolecular (ortho‑nitrophenol).

10. State functions vs path functions

State: internal energy, Gibbs energy. Path: heat, work.

11. Common ion effect with example

Dissociation of CH₃COOH suppressed by adding CH₃COONa (common acetate ion).

12. Disproportionation reaction – example

Same substance oxidized & reduced.
e.g. 2H₂O₂ → 2H₂O + O₂.

13. Detection of sulphur (sodium fusion extract)

Add sodium nitroprusside → violet colour confirms S.

14. Aromatic according to Hückel’s rule

(Depends on image) must be cyclic, planar, conjugated, (4n+2)π electrons.

15. (a) CH₃Cl + anhyd. AlCl₃ → ?
(b) C₂H₅Br + 2Na + BrC₂H₅ (dry ether) → ?

(a) Toluene (Friedel‑Crafts); (b) Butane (Wurtz).

✦ Questions 16 – 26 (answer any eight, 3 scores each)

16. Define: atomic mass unit, limiting reagent, molarity

a.m.u. = 1/12 mass of ¹²C; limiting reagent = reactant completely consumed; molarity = mol solute / L solution.

17. Photoelectric effect – results

Threshold frequency, K.E. ∝ frequency (not intensity), intensity → number of e⁻.

18. (i) Ionization enthalpy down a group? (ii) Be, Mg, Ca: highest IE & most metallic

(i) Decreases (size ↑, shielding ↑). (ii) Highest IE: Be; most metallic: Ca.

19. Electron gain enthalpy; F vs Cl more negative?

Electron gain enthalpy = energy released adding e⁻ to atom. Cl has more negative EGA than F (small F has high e⁻ density).

20. MO configuration of N₂, bond order, magnetism

(σ1s)²(σ*1s)²(σ2s)²(σ*2s)²(π2p)⁴(σ2p)² ; bond order = 3 ; diamagnetic.

21. Δ_fH of CH₄ (using Hess’s law)

-75 kJ mol⁻¹ (from combustion data).

22. Buffer solutions – types with examples

Acidic: CH₃COOH/CH₃COONa; Basic: NH₄OH/NH₄Cl. Resist pH change.

23. Balance in basic medium: MnO₄⁻ + I⁻ → MnO₂ + I₂

2MnO₄⁻ + 4H₂O + 6I⁻ → 2MnO₂ + 3I₂ + 8OH⁻

24. Metamers of C₄H₁₀O (ethers) with IUPAC

Ethoxyethane (C₂H₅OC₂H₅), 1‑methoxypropane (CH₃OC₃H₇), 2‑methoxypropane (CH₃OCH(CH₃)₂).

25. Sawhorse eclipsed / staggered ethane; stability

Staggered more stable (less torsional strain). [Sawhorse sketches described].

26. CH₃–CH=CH₂ + (i) H₂/Ni ; (ii) O₃ then Zn/H₂O

(i) Propane; (ii) Ethanal + methanal (CH₃CHO + HCHO).

✦ Questions 27 – 31 (answer any four, 4 scores each)

27. Information from four quantum numbers

n (shell), l (subshell shape), mₗ (orbital orientation), mₛ (spin).

28. VSEPR postulates; shapes of BF₃, NH₃, H₂O

Postulates: e⁻ pairs repel; geometry minimizes repulsion.
BF₃: trigonal planar; NH₃: pyramidal; H₂O: bent.

29. Define enthalpy, entropy; spontaneous ΔG sign

Enthalpy (H) = U+PV; entropy (S) = disorder; spontaneous: ΔG < 0.

30. (i) Pressure & temp effect on N₂+3H₂⇌2NH₃ (exothermic)
(ii) pH of 0.02 M HCl

(i) ↑P favours forward (fewer moles); ↑T favours reverse.
(ii) pH = –log(0.02) = 1.70

31. (i) Electromeric & inductive effect; (ii) homolytic vs heterolytic fission

Electromeric: temporary π‑shift; inductive: permanent σ‑polarisation.
Homolytic: symmetric → radicals; heterolytic: asymmetric → ions.

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⚛️ Note: All equations, sketches and structural formulae should be shown in the original answer sheet. This key provides concise expected answers based on the question paper.

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📐 p orbital sketch: two lobes, nodal plane

🧪 sodium fusion test: violet with nitroprusside

📌 Q15 detailed:

(a) CH₃Cl + C₆H₆ --Anhyd. AlCl₃--> C₆H₅CH₃ (toluene) [Friedel‑Crafts alkylation]
(b) 2C₂H₅Br + 2Na --ether--> C₄H₁₀ (butane) + 2NaBr [Wurtz reaction]

✦ Answer key v1.0 – for instructional use only. Numerical values verified.