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HW1solutions

# HW1solutions - Niu(qn269 Homework 1 owers(53885 This...

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Niu (qn269) – Homework 1 – flowers – (53885) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points What is responsible for the abnormally high melting and boiling points of water? 1. covalent bonding 2. hydrogen bonding correct 3. dispersion bonding 4. ionic bonding Explanation: H-bonds are a special case of very strong dipole-dipole interactions. They only occur when H is bonded to small, highly electroneg- ative atoms – F, O or N only. 002 10.0 points The high heat capacity of water explains: 1. the dramatic temperature changes on the moon between day and night correct 2. the reason water is considered the univer- sal solvent 3. the blue color of the oceans and sky 4. the reason ice is less dense than water 5. the hydrogen bonding observed to occur between water molecules Explanation: 003 10.0 points The vapor pressure of CH 3 COCH 3 (propa- none or acetone), is 67 Torr at 0 C and 222 Torr at 25 C. What is the normal boiling point of propanone? The universal gas con- stant is 8 . 314 J K · mol . Correct answer: 55 . 941 C. Explanation: T 1 = 0 C + 273 . 15 = 273 . 15 K T 2 = 25 C + 273 . 15 = 298 . 15 K P 1 = 67 Torr P 2 = 222 Torr P 0 = 1 atm = 760 Torr R = 8 . 314 J K · mol Using the Clausius-Clapeyron equation, ln parenleftbigg P 1 P 2 parenrightbigg = Δ H vap R parenleftbigg 1 T 2 - 1 T 1 parenrightbigg R ln parenleftbigg P 1 P 2 parenrightbigg 1 T 2 - 1 T 1 = Δ H vap Δ H vap = ( 8 . 314 J K · mol ) ln parenleftbigg 67 Torr 222 Torr parenrightbigg 1 298 . 15 K - 1 273 . 15 K = 32445 . 7 J / mol . Applying the Clausius-Clapeyron equation again, ln parenleftbigg P 0 P 2 parenrightbigg = Δ H vap R parenleftbigg 1 T 2 - 1 T 0 parenrightbigg R Δ H vap ln parenleftbigg P 0 P 2 parenrightbigg = 1 T 2 - 1 T 0 1 T 0 = 1 T 2 - R Δ H vap ln parenleftbigg P 0 P 2 parenrightbigg = 1 298 . 15 K - 8 . 314 J K · mol 32445 . 7 J / mol ln parenleftbigg 760 Torr 222 Torr parenrightbigg = 0 . 00303867 C T 0 = 1 0 . 00303867 C = 329 . 091 C - 273 . 15 = 55 . 941 K 004 10.0 points Some of the following terms characterize both the bonding within a molecule ( in- tra molecular) and that between atoms and molecules ( inter molecular). Which of the fol- lowing is normally considered only when char- acterizing inter molecular forces?

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Niu (qn269) – Homework 1 – flowers – (53885) 2 1. electrostatic forces 2. polar covalent bonding 3. covalent bonding 4. van der Waals forces correct 5. ionic forces Explanation: Only van der Waals forces occur between separate molecules; i.e. , are inter molecular forces. 005 10.0 points Which of the following molecules A) CH 3 OCH 3 C) CH 3 CHO B) CH 3 COOH D) CH 3 CH 2 OH are likely to form hydrogen bonds? 1. B and D only correct 2. None forms hydrogen bonds. 3. A and D only 4. All are likely to form hydrogen bonds. 5. A and B only 6. Another combination of compounds 7. C and D only 8. A and C only 9. B and C only Explanation: Only molecules with H attached to the electronegative atoms N, O, and F can hy- drogen bond. Of the molecules given, only CH 3 COOH and CH 3 CH 2 OH have hydrogen attached to oxygen, so these are the only ones that can undergo hydrogen bonding.
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HW1solutions - Niu(qn269 Homework 1 owers(53885 This...

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