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Unformatted text preview: wilson (tw8656) H08 : VSEPR mccord (50960) 1 This print-out should have 31 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which of the following statements about po- larity is false? 1. CCl 4 is a polar molecule. correct 2. Lone (unshared) pairs of electrons on the central atom play an important role in influ- encing polarity. 3. Linear molecules can be polar. 4. Polar molecules must have a net dipole moment. 5. Dipole moments can cancel, giving a net non-polar molecule. Explanation: The Lewis Dot structure for CCl 4 is C Cl Cl Cl Cl The molecule has tetrahedral electronic and molecular geometry. The C- Cl bond is polar, but becuase of the symmetry of the molecule, the individual dipole moments cancel. The molecule is therefore nonpolar. 002 10.0 points Which of the following molecules is nonpolar? 1. BF 3 correct 2. NF 3 3. SO 2 4. CH 3 Br 5. H 2 O Explanation: The structure for BF 3 is B F F F There are no lone pairs on B. This is a symmetrical molecule and non-polar. 003 10.0 points CHF 3 is (less,more) polar than CHI 3 because 1. less; the tetrahedral geometry decreases the polarity of C-F bonds. 2. less; the C-H bond in CHF 3 is a non-polar bond. 3. less; the three polar C-F bonds are sym- metrical and cancel the dipole moments. 4. more; the C-H bond in CHF 3 is a non- polar bond. 5. more; the C-F bonds are more polar than the C-I bonds. correct Explanation: Since F is more electronegative than I, a C-F bond will have a greater difference in electronegativities than a C-I bond. 004 10.0 points Which of the molecules I) O b b b b H H II) B Cl b b b b b b Cl b b b b b b Cl b b b b b b III) O b b b b C O b b b b contains polar covalent bonds but is NOT itself a polar molecule? 1. II only wilson (tw8656) H08 : VSEPR mccord (50960) 2 2. I only 3. I and II only 4. II and III only correct 5. All fit the criteria. 6. III only 7. I and III only 8. None fit the criteria. Explanation: I) Water has polar covalent bonds and is itself polar. II) Boron trichloride contains polar covalent bonds but is not polar overall due to symmetry. III) Carbon dioxide contains polar covalent bonds but is not polar overall due to symmetry. 005 10.0 points Consider a 3-atom molecule A B A for which B has a total of only four valence elec- trons enough to make two bonds. Predict the A-B-A bond angle. 1. 109.5 2. 90 3. 60 4. 180 correct 5. 120 Explanation: 006 10.0 points What is the shape of COCl 2 ? 1. trigonal planar correct 2. T-shaped 3. trigonal pyramidal 4. tetrahedral 5. seesaw Explanation: There are three regions of electron density (with no lone pairs) around the central atom: C Cl b b b b b b Cl b b b b b b O b b b b 007 10.0 points Which of the following has bond angles slightly LESS than 120 ?...
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- Fall '07