PMC National MDCAT Chemistry Chemical Bonding — Set 2

1. Question 1

   Q1. A molecule of Hydrogen Fluoride (HF) has a bond length of 91.7 pm and a dipole moment of 1.91 D. Calculate the charge in units of 10^-10 esu.

   • A) 2.08
   • B) 1.91
   • C) 4.80
   • D) 0.48

   Answer: 2.08

   Explanation: HF dipole moment is 1.91 D; dividing this by the bond length in cm (0.917e-8) yields 2.08e-10 esu. 4.80 is the electronic charge.

2. Question 2

   Q2. In a methane (CH4) molecule, the four C-H bonds are equivalent and tetrahedral. What is the total sum of all H-C-H bond angles in degrees?

   • A) 438.0
   • B) 657.0
   • C) 328.5
   • D) 109.5

   Answer: 657.0

   Explanation: A tetrahedron has six H-C-H bond angles of 109.5 degrees each, totaling 657 degrees. 438 only accounts for four angles.

3. Question 3

   Q3. A sample of vinyl acetylene (C4H4) is analyzed for its bonding characteristics. How many sigma bonds are present in one molecule of this compound?

   • A) 3
   • B) 4
   • C) 7
   • D) 11

   Answer: 7

   Explanation: Vinyl acetylene (CH2=CH-C≡CH) has 3 C-C sigma bonds and 4 C-H sigma bonds. 11 represents the total of sigma and pi bonds.

4. Question 4

   Q4. If the electronegativity difference between two atoms forming a chemical bond is 1.7, what is the approximate percentage of ionic character in that bond?

   • A) 15%
   • B) 30%
   • C) 50%
   • D) 72%

   Answer: 50%

   Explanation: A difference of 1.7 on the Pauling scale indicates 50% ionic character. 72% corresponds to a much higher electronegativity difference of 2.1.

5. Question 5

   Q5. The bond energy of H-H is 436 kJ/mol and Cl-Cl is 242 kJ/mol. If the enthalpy of formation of HCl is -92 kJ/mol, calculate the H-Cl bond energy.

   • A) 339
   • B) 431
   • C) 678
   • D) 184

   Answer: 431

   Explanation: Using Hess's law, BE(H-Cl) = 0.5*BE(H-H) + 0.5*BE(Cl-Cl) - dHf. 431 kJ/mol is the result. 339 fails by ignoring the enthalpy of formation.

6. Question 6

   Q6. In the Born-Haber cycle for NaCl, sublimation is +108, IE is +496, Cl dissociation is +242, EA is -349, and Lattice Energy is -787. Calculate dHf.

   • A) -411
   • B) +411
   • C) -311
   • D) -811

   Answer: -411

   Explanation: dHf = Sublimation + IE + 0.5*Dissociation + EA + Lattice Energy. (108+496+121-349-787) = -411. +411 incorrectly assumes an endothermic overall process.

7. Question 7

   Q7. A diatomic molecule has a dipole moment of 1.2 D and a bond length of 1.0 Angstrom. Calculate the fractional charge on the atoms.

   • A) 0.12
   • B) 0.25
   • C) 0.50
   • D) 0.75

   Answer: 0.25

   Explanation: Fractional charge is calculated as observed dipole (1.2) divided by calculated dipole (4.8). 1.2/4.8 equals 0.25. 0.50 suggests a much higher dipole.

8. Question 8

   Q8. In a molecule of hydrogen sulfide (H2S), the central atom contains two lone pairs. Calculate the total number of valence electrons used in these lone pairs.

   • A) 2
   • B) 4
   • C) 6
   • D) 8

   Answer: 4

   Explanation: H2S has two lone pairs on the Sulfur atom; since each pair has 2 electrons, the total is 4. 2 is only the number of pairs.

9. Question 9

   Q9. According to Molecular Orbital Theory, the bond order of the O2+ ion is calculated using its electron configuration. What is the resulting bond order?

   • A) 1.5
   • B) 2.0
   • C) 2.5
   • D) 3.0

   Answer: 2.5

   Explanation: O2+ has 10 bonding and 5 antibonding electrons; (10-5)/2 = 2.5. 2.0 is the bond order for neutral O2, which has 6 antibonding electrons.

10. Question 10

    Q10. A central atom undergoes sp2 hybridization to form a planar molecule. What is the sum of all three interior bond angles in that plane?

    • A) 180
    • B) 360
    • C) 540
    • D) 720

    Answer: 360

    Explanation: A trigonal planar molecule (sp2) has three 120-degree angles in one plane; 120 times 3 equals 360. 180 only accounts for 1.5 angles.

11. Question 11

    Q11. An organic compound has the structural formula CH2=C=CH2. What is the total number of sp2 hybridized carbon atoms present in this specific molecule?

    • A) 1
    • B) 2
    • C) 3
    • D) 0

    Answer: 2

    Explanation: In allene (CH2=C=CH2), the terminal carbons are sp2 hybridized while the central carbon is sp. 3 incorrectly includes the central carbon.

12. Question 12

    Q12. In the Lewis structure of the ammonium ion (NH4+), what is the calculated formal charge on the nitrogen atom based on its valence electrons?

    • A) -1
    • B) 0
    • C) +1
    • D) +5

    Answer: +1

    Explanation: Formal charge = Valence electrons - (Unshared + 0.5*Shared). 5 - (0 + 4) = +1. 0 would imply a neutral atom with five bonds.

13. Question 13

    Q13. The C-C bond length in ethane is 1.54 Angstroms and in ethene is 1.34 Angstroms. What is the numerical difference in these lengths in picometers?

    • A) 0.2
    • B) 2
    • C) 20
    • D) 200

    Answer: 20

    Explanation: The difference is 1.54 - 1.34 = 0.20 Angstroms. Since 1 Angstrom = 100 pm, 0.20 Angstroms equals 20 pm. 200 is a factor error.

14. Question 14

    Q14. A student reacts boron trifluoride with ammonia to form an adduct. How many total coordinate covalent bonds are present in two moles of this adduct?

    • A) 3.01 x 10^23
    • B) 6.02 x 10^23
    • C) 1.20 x 10^24
    • D) 2.40 x 10^24

    Answer: 1.20 x 10^24

    Explanation: One mole of BF3-NH3 contains 6.02e23 coordinate bonds; two moles contain 1.20e24. 6.02e23 is only for one mole of the adduct.

15. Question 15

    Q15. In the carbonate ion (CO3^2-), multiple resonance structures can be drawn. What is the total number of valence electrons used in its Lewis structure?

    • A) 20
    • B) 22
    • C) 24
    • D) 26

    Answer: 24

    Explanation: Carbon (4) + 3 Oxygens (18) + Charge (2) = 24 valence electrons. 22 fails to account for the -2 charge of the ion.

16. Question 16

    Q16. According to the Pauling scale, Oxygen has an electronegativity of 3.5 and Hydrogen has 2.1. What is the numerical difference that determines water's polarity?

    • A) 0.4
    • B) 0.9
    • C) 1.4
    • D) 5.6

    Answer: 1.4

    Explanation: The electronegativity difference is 3.5 - 2.1 = 1.4. 0.9 is the difference for C-H bonds, not O-H bonds.

17. Question 17

    Q17. The total energy required to dissociate one mole of gaseous methane (CH4) into gaseous atoms is 1664 kJ/mol. Calculate the average C-H bond energy.

    • A) 416
    • B) 832
    • C) 1664
    • D) 436

    Answer: 416

    Explanation: Methane has four C-H bonds; 1664 kJ/mol divided by 4 yields 416 kJ/mol per bond. 436 is the bond energy of H-H molecules.

18. Question 18

    Q18. In a PCl5 molecule, the axial P-Cl bonds are longer than the equatorial ones. If axial is 240 pm and equatorial is 202 pm, calculate the difference.

    • A) 19
    • B) 38
    • C) 48
    • D) 58

    Answer: 38

    Explanation: The difference between axial (240 pm) and equatorial (202 pm) bonds in PCl5 is 38 pm. 19 is exactly half of the correct difference.

19. Question 19

    Q19. Carbon tetrachloride (CCl4) has four polar C-Cl bonds, each with a dipole of 1.5 D. What is the resultant dipole moment of the whole molecule?

    • A) 0
    • B) 1.5
    • C) 3.0
    • D) 6.0

    Answer: 0

    Explanation: CCl4 is a perfectly symmetrical tetrahedron; the vector sum of four identical bond dipoles is zero. 6.0 incorrectly sums them as scalars.

20. Question 20

    Q20. A molecule of benzene (C6H6) contains a delocalized system. Calculate the ratio of total sigma bonds to total pi bonds in one molecule.

    • A) 1:1
    • B) 2:1
    • C) 3:1
    • D) 4:1

    Answer: 4:1

    Explanation: Benzene has 12 sigma bonds (6 C-C, 6 C-H) and 3 pi bonds; the ratio 12:3 simplifies to 4:1. 2:1 ignores the C-H sigma bonds.