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Section Time:
LS1a
ICE 1
Name:
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1. (5 points) Draw the maximum number of water molecules to which the water molecule shown
below can simultaneously form hydrogen bonds. Indicate these hydrogen bonds using a dashed line
(
). All hydrogen bonds should be drawn with hydrogen bond angles.
2. (5 points) Shown below are the structures of two toxic gases, hydrogen cyanide and carbon
monoxide. Assign formal charges to the atoms in each of these molecules. All lone pair electrons
that are present have been indicated for you. It is possible that a molecule contains no atoms with
formal charges. [1 point per atom]
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3. (10 points) The structures of pentane, isopentane, and neopentane are shown below. While all
three molecules have the same molecular formula, C5H12, their boiling points vary.
a. (3 points) The boiling points for these three molecules are listed below in no particular order. In
the blank next to each boiling point value, write the name of the molecule (neopentane,
isopentane, or pentane) to which it corresponds.
Boiling Point
Molecule
9.5°C
Neopentane_____________
27.7°C
Isopentane______________
36.0°C
Pentane_________________
b. (7 points) Using your knowledge of intermolecular interactions, briefly explain why these three
molecules have different boiling points despite having the same molecular weight.
For a molecule to boil, its intermolecular interactions must be broken; therefore, a higher boiling
point is associated with stronger and/or more numerous intermolecular interactions.
Since these molecules in questions (neopentane, isopentane, or pentane) are non-polar, they can
only interact with one another via van der Waals interactions.
The amount of van der Waals interactions that can be formed between these molecules is
proportional to the exposed surface area of the molecule; pentane has an extended geometry
with a lot of surface area, enabling it to form more van der Waals interactions than neopentane,
which has a compact, globular shape that features less exposed surface area.
Do not write below this line.
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