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exam 2 - Version 005 Exam 2 mcdevitt(52365 This print-out...

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Version 005 – Exam 2 – mcdevitt – (52365) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. You are responsible for answering 25 ques- tions correctly. There are two extra credit questions. 001 10.0 points Which of the following is the correct Lewis formula for carbon dioxide (CO 2 )? 1. O C O 2. O C O 3. O C O 4. O C O 5. O C O correct 6. O C O 7. O C O 8. O C O 9. O C O 10. O C O Explanation: C contributes 4 valence e and each O con- tributes 6 valence e for a total of 16 e . The Lewis formula for carbon dioxide (CO 2 ) is O C O 002 10.0 points An ionic bond is formed when electrons are 1. split. 2. transferred. correct 3. destroyed. 4. shared. Explanation: 003 10.0 points Which of the following molecules is polar? 1. CH 4 2. NH 3 correct 3. CF 4 4. H 2 5. BH 3 Explanation: CH 4 and CF 4 are both tetrahedral and sym- metric. Polar bonds will cancel. H 2 is a di- atomic molecule and is not polar. Both Hs have the same EN and therefore there is no ΔEN and the bond is not polar. BH 3 is trigo- nal planar and symmetric. There are no lone pairs because B is an exception to the octet rule. The 3 B H bonds cancel each other. NH 3 has 3 polar N H bonds and 1 lone pair on N. NH 3 has tetrahedral electronic geome- try and trigonal pyramidal geometry. The molecule is not symmetric, the N H bonds polarities do not cancel, and it is therefore a polar molecule. 004 10.0 points How many polar covalent bonds are in NH 3 ? 1. 4 2. 3 correct 3. 1 4. None of these 5. 2 Explanation: To draw the dot structure for NH 3 we must first calculate the number of valence electrons
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Version 005 – Exam 2 – mcdevitt – (52365) 2 available from the atoms. A = 5 × 1 (N atom) + 1 × 3 (H atoms) = 8 e Hydrogen can form only one bond, so nitro- gen must be the central atom. We place the other atoms symmetrically around the nitro- gen atom. The correct dot structure for the molecule should show a complete octet (8 electrons) around the nitrogen atom and two electrons around each hydrogen atom, a total of 8 va- lence electrons for the entire structure: N ·· H H H There are 3 bonds in the structure. Nitrogen-hydrogen bonds, bonds between two non-metal atoms, will be covalent. Since nitrogen and hydrogen have different elec- tronegativities, these bonds will be polar. (If using the S = N - A rule to determine the dot structure, N = 8 × 1 + 2 × 3 = 14 e and S = 14 - 8 = 6 e . This would indicate 3 bonds and correspond to the structure shown above.) 005 10.0 points What is the hybridization of the S atom in SF 6 ?
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