ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Chemical Bonding (Set 7): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer review. Covers 30…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Arrange the C–O bond lengths of carbon monoxide (CO), carbon dioxide (CO₂) and the carbonate ion (CO₃²⁻) in increasing order.
Answer: B. CO < CO₂ < CO₃²⁻
Explanation: Bond length falls as bond order rises. The C–O bond order is 3 in CO, 2 in CO₂ and 4/3 in CO₃²⁻ (resonance), so the lengths increase CO < CO₂ < CO₃²⁻.
Q2. Which of the following species is diamagnetic in its ground state (use molecular orbital theory; valence electrons: O₂ 12, O₂⁻ 13, N₂⁺ 9, F₂ 14)?
Answer: A. F₂
Explanation: F₂ has all electrons paired (…π*2p⁴). O₂ (π*2p², two unpaired), O₂⁻ (one unpaired) and N₂⁺ (one unpaired) are paramagnetic.
Q3. The shape of the chlorate ion, ClO₃⁻ (central Cl with three bond pairs and one lone pair), is:
Answer: D. trigonal pyramidal
Explanation: Four electron pairs give tetrahedral electron geometry; with one lone pair the molecular shape is trigonal pyramidal.
Q4. Among BeCl₂, BF₃, CH₄, NH₃, H₂O and CO₂, how many have a bond angle strictly greater than 109.5°?
Answer: D. 3
Explanation: BeCl₂ (180°), BF₃ (120°) and CO₂ (180°) exceed 109.5°. CH₄ is exactly 109.5°, NH₃ is 107° and H₂O is 104.5° (lone-pair repulsion). So 3 species.
Q5. The hybridisation of the nitrogen atom in the nitronium ion, NO₂⁺ (O=N=O)⁺, is:
Answer: A. sp
Explanation: Nitrogen has two σ bonds and no lone pair (two electron domains), so it is sp hybridised and the ion is linear.
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Q6. In acrylonitrile, CH₂=CH–C≡N, the numbers of σ bonds and π bonds are respectively:
Q7. Among cis-1,2-dichloroethene, trans-1,2-dichloroethene and 1,1-dichloroethene, which one has zero dipole moment?
Q8. The dipole moments of the three isomeric dichlorobenzenes follow the order:
Q9. Lattice energy of LiF from the Born–Haber cycle, using ΔH_f = −617, sublimation of Li = +159, ½ bond dissociation of F₂ = +79, ionisation energy of Li = +520 and electron affinity of F = −328 (all kJ mol⁻¹), is:
Q10. For CH₃–H + Cl–Cl → CH₃–Cl + H–Cl use mean bond enthalpies (kJ mol⁻¹): C–H 413, Cl–Cl 243, C–Cl 339, H–Cl 432. The enthalpy change is:
Q11. In the formate ion, HCOO⁻, the two C–O bonds are identical because of resonance. The C–O bond order is:
Q12. In the Lewis structure of ozone, O=O⁺–O⁻, the formal charge on the central oxygen atom is:
Q13. Which of the following ionic solids is expected to have the highest melting point?
Q14. Which of the following chlorides has the greatest covalent character (according to Fajans' rules)?
Q15. In ammonium tetrafluoroborate, NH₄BF₄, count the coordinate covalent (dative) bonds inside the NH₄⁺ ion and inside the BF₄⁻ ion (the ionic bond between the two ions is not counted). The total is:
Q16. The C–H bond is shortest in which of the following hydrocarbons?
Q17. According to molecular orbital theory, the number of electrons in antibonding valence molecular orbitals (σ*2s and π*2p) of an O₂ molecule is:
Q18. Which of the following carbon–carbon bonds is the longest?
Q19. Magnesium metal has a higher melting point than sodium because, in magnesium:
Q20. Which of the following hydrogen bonds is the strongest?
Q21. A water molecule has an O–H bond dipole of 1.5 D and an H–O–H angle of 104.5° (cos 52.25° = 0.6124). The resultant dipole moment of the molecule is:
Q22. A hydrogen 1s orbital approaches a p_x orbital of another atom along the z-axis (internuclear axis). The overlap is:
Q23. The best Lewis structure of the nitrate ion, NO₃⁻, shows how many lone pairs on all atoms combined (24 valence electrons; four bonding pairs including one π bond)?
Q24. When ammonia reacts with boron trifluoride, H₃N → BF₃ is formed. The hybridisation of boron changes from:
Q25. Which of the isoelectronic ions O²⁻, F⁻, Na⁺ and Mg²⁺ has the largest radius?
Q26. Which of the following molecules has the largest H–X–H bond angle?
Q27. The total number of σ bonds in 1,3-butadiene, CH₂=CH–CH=CH₂, is:
Q28. Solid sodium chloride does not conduct electricity but molten sodium chloride does. This is because in the molten state:
Q29. The Cl–Cl bond (243 kJ mol⁻¹) is stronger than the F–F bond (159 kJ mol⁻¹), even though fluorine is smaller. The accepted reason is:
Q30. In the molecule CH₃–CH=CH–C≡CH, the numbers of sp³, sp² and sp hybridised carbon atoms are respectively: