ECAT — Engineering College Admission Test (UET)

ECAT Chemistry — Chemical Bonding (Set 4)

Take the ECAT Chemistry — Chemical Bonding (Set 4): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer review. Covers 30…

What this test covers

30 ECAT Chemistry

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. According to molecular orbital theory, how many unpaired electrons are present in the ground state of the B₂ molecule?
- A. 0
- B. 2 ✓
- C. 4
- D. 1
Answer: B. 2
Explanation: For B₂ (10 electrons) the π2p orbitals lie below σ2p because of s–p mixing, so the last two electrons occupy the two degenerate π2p orbitals singly (Hund's rule). B₂ is paramagnetic with 2 unpaired electrons.
Q2. In the MO diagram of the carbide ion C₂²⁻ (isoelectronic with N₂), how many electrons occupy antibonding molecular orbitals, counting all occupied orbitals including those from 1s?
- A. 0
- B. 4 ✓
- C. 6
- D. 2
Answer: B. 4
Explanation: C₂²⁻ has 12 + 2 = 14 electrons, like N₂: σ1s², σ*1s², σ2s², σ*2s², π2p⁴, σ2p². Antibonding electrons = 4, bonding = 10, bond order 3.
Q3. The ion ICl₄⁻ has how many lone pairs on the central atom, and what is its shape?
- A. 1; square pyramidal
- B. 0; octahedral
- C. 2; tetrahedral
- D. 2; square planar ✓
Answer: D. 2; square planar
Explanation: Iodine has 7 + 1 = 8 valence electrons; four go to bonds, leaving 4 electrons = 2 lone pairs. Six electron pairs (sp³d²) with 2 lone pairs opposite each other give a square planar shape.
Q4. The dipole moment of HCl is 1.03 D and its bond length is 127 pm. Taking the dipole moment of a fully ionic H⁺Cl⁻ as e × d (e = 4.8×10⁻¹⁰ esu, 1 D = 10⁻¹⁸ esu cm), the percentage ionic character of HCl is about
- A. 8%
- B. 51%
- C. 17% ✓
- D. 34%
Answer: C. 17%
Explanation: Fully ionic μ = 4.8 × 1.27 = 6.1 D. Ionic character = 1.03/6.1 × 100 ≈ 17%.
Q5. In which dichloroethene do the two C–Cl bond dipoles cancel to give a zero net dipole moment?
- A. chloroethene (vinyl chloride)
- B. cis-1,2-dichloroethene
- C. 1,1-dichloroethene
- D. trans-1,2-dichloroethene ✓
Answer: D. trans-1,2-dichloroethene
Explanation: In the trans isomer the two C–Cl bond dipoles are equal and opposite and cancel. In the cis and 1,1- isomers they add, and vinyl chloride has a single C–Cl dipole.

The other 25 questions

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Q6. Which silver halide has the greatest covalent character?
- A. AgI
- B. AgCl
- C. AgBr
- D. AgF
Q7. Which of the sodium halides NaF, NaCl, NaBr and NaI has the lowest melting point?
- A. NaBr
- B. NaF
- C. NaCl
- D. NaI
Q8. In the molecule vinylacetylene, CH₂=CH–C≡CH, what is the ratio of sigma to pi bonds?
- A. 8 : 2
- B. 6 : 4
- C. 5 : 5
- D. 7 : 3
Q9. What are the shapes of the species CH₃⁺ and CH₃⁻?
- A. both trigonal pyramidal
- B. both trigonal planar
- C. CH₃⁺ trigonal planar; CH₃⁻ trigonal pyramidal
- D. CH₃⁺ trigonal pyramidal; CH₃⁻ trigonal planar
Q10. Which of the hydrides PH₃, NH₃, CH₄ and H₂O has a bond angle closest to 90°?
- A. CH₄
- B. PH₃
- C. NH₃
- D. H₂O
Q11. The F–N–F angle in NF₃ (about 102°) is smaller than the H–N–H angle in NH₃ (about 107°). The best explanation is that
- A. NF₃ has two lone pairs on nitrogen while NH₃ has only one, so lone pair–bond pair repulsion is larger in NF₃
- B. nitrogen is sp² hybridized in NF₃ but sp³ in NH₃, which makes the ideal bond angle smaller
- C. fluorine atoms are larger than hydrogen atoms, so steric crowding between the F atoms squeezes the bond angle down
- D. the highly electronegative fluorine atoms draw the bonding electrons away from nitrogen, reducing bond-pair repulsion
Q12. How many coordinate (dative) covalent bonds are present in the dimer Al₂Cl₆?
- A. 6
- B. 2
- C. 0
- D. 4
Q13. Among Na, Mg, Al and Si, which element has the highest second ionization energy?
- A. Al
- B. Si
- C. Na
- D. Mg
Q14. Which list gives the correct order of decreasing polarity of the bonds H–F, H–O, H–N, H–C?
- A. H–C > H–N > H–O > H–F
- B. H–O > H–F > H–N > H–C
- C. H–F > H–O > H–N > H–C
- D. H–F > H–N > H–O > H–C
Q15. o-Nitrophenol is steam volatile but p-nitrophenol is not. This is because o-nitrophenol has
- A. a higher molecular mass than the para isomer
- B. stronger ionic character
- C. intermolecular hydrogen bonding between molecules
- D. intramolecular hydrogen bonding, so molecules do not associate
Q16. The melting points of the metals follow the order Na < Mg < Al. The main reason is that
- A. the ionization energy decreases across the period, so the atoms release valence electrons more easily
- B. the atomic mass increases across the period, so heavier atoms are held more strongly in the lattice
- C. the number of delocalized electrons per atom and the ionic charge increase across the period
- D. the number of neutrons increases across the period, strengthening the metallic bond between atoms
Q17. Removal of one electron from N₂ and from O₂ gives N₂⁺ and O₂⁺. Which statement about the bond strength is correct?
- A. N₂⁺ has a stronger bond than N₂, but O₂⁺ has a weaker bond than O₂
- B. Both N₂⁺ and O₂⁺ have weaker bonds than their neutral molecules
- C. N₂⁺ has a weaker bond than N₂, but O₂⁺ has a stronger bond than O₂
- D. Both N₂⁺ and O₂⁺ have stronger bonds than their neutral molecules
Q18. The C–C bond length in graphite (142 pm) is shorter than that in diamond (154 pm) because
- A. each graphite C–C bond has partial double-bond character from delocalized π electrons
- B. graphite carbon atoms use d orbitals in bonding, which contracts the C–C bond relative to diamond
- C. graphite carbon atoms are sp³ hybridized, giving shorter and stronger tetrahedral bonds than diamond
- D. graphite has ionic bonding between its layers, which pulls the carbon atoms of each sheet closer together
Q19. Which species has a square pyramidal shape?
- A. XeF₄
- B. SF₄
- C. BrF₅
- D. PF₅
Q20. Which statement about the ozone molecule O₃ is correct?
- A. It is linear with two equal double bonds
- B. Both O–O bonds are equal in length, with a bond order of 1.5
- C. One O–O bond is a double bond and the other a single bond with different lengths
- D. It has a bond order of 2 for each O–O bond
Q21. Which of the following species has an expanded octet around its central atom?
- A. NO
- B. BeCl₂
- C. BF₃
- D. SF₆
Q22. Each O–H bond in water has a bond dipole of 1.5 D and the H–O–H angle is 104.5°. The resultant dipole moment of the molecule is about
- A. 0.92 D
- B. 1.84 D
- C. 2.37 D
- D. 3.00 D
Q23. A bent molecule AB₂ has two identical A–B bond dipoles of 1.2 D and a B–A–B angle of 120°. What is the molecular dipole moment?
- A. 1.2 D
- B. 0.6 D
- C. 2.08 D
- D. 2.4 D
Q24. Which of the following has the smallest ionic radius?
- A. F⁻
- B. O²⁻
- C. Mg²⁺
- D. Na⁺
Q25. Calculate the lattice energy of KF from these data (kJ mol⁻¹): ΔHf°(KF) = −568; sublimation of K = +89; ionization energy of K = +419; ½ bond dissociation of F₂ = +79; electron affinity of F = −328.
- A. −309 kJ mol⁻¹
- B. −568 kJ mol⁻¹
- C. −1395 kJ mol⁻¹
- D. −827 kJ mol⁻¹
Q26. Which is the correct order of bond dissociation energy of the halogen molecules?
- A. F₂ > Cl₂ > Br₂ > I₂
- B. I₂ > Br₂ > Cl₂ > F₂
- C. Br₂ > Cl₂ > F₂ > I₂
- D. Cl₂ > Br₂ > F₂ > I₂
Q27. How many nodal planes in total does the π* (antibonding) molecular orbital formed from 2p orbitals have?
- A. 3
- B. 2
- C. 0
- D. 1
Q28. Which anion is the most easily polarized?
- A. S²⁻
- B. F⁻
- C. O²⁻
- D. Cl⁻
Q29. Which of these can accept hydrogen bonds from water but cannot form hydrogen bonds between its own molecules?
- A. methanol, CH₃OH
- B. ammonia, NH₃
- C. ethanoic acid, CH₃COOH
- D. dimethyl ether, CH₃OCH₃
Q30. Which is the correct order of C–H bond length in ethane, ethene and ethyne?
- A. ethane < ethene < ethyne
- B. ethene < ethane < ethyne
- C. ethyne < ethene < ethane
- D. ethane < ethyne < ethene