Name:_____________
Chemistry 114
Second Hour Exam
Remember- Show all work for partial credit
1. (12 points) X-rays with a wavelength of 1.54 Å were used to analyze an aluminum
crystal. If the aluminum atoms in the crystal had a spacing of 2.33 Å, what was the
angle of reflection for the X-rays striking the crystal?
n8=2d sin2
1(1.54Å) = 2(2.33Å) sin 2
1.54Å/4.66Å = sin 2
.3305=sin2
2=19.3o
2. (13 points) Fil in the following table on solids
Group 8A
Structural
unit (Atoms,
molecules,
Ions,
Polymers,
Amorphous)
Atom
Network
Atom
Type of
Bonding
Melting
point:(High,
Low, wide
range)
Example:
Metallic
Molecular
Ionic
Molecule
Ion
Metallic or
nondirectional
covalent
Dipole and
London
ChargeCharge
Atom
London
Directional
Covalent
Low
High
High or
range
range
High
Ne
diamond
Iron
water
NaCl
2
3. (12 points) I have isolated a new molecule that has a normal boiling point of 1200 C,
and a heat of vaporization of 50 kJ/mole. What is the vapor pressure of this substance
at room temperature, 25oC?
4. (13 points) Consider the following melting point data:
Compound:
NaCl MgCl2 AlCl3 SiCl4 PCl3 SCl2
o
Melting point ( C): 801 708 190 -70
-91
-78
Compound:
NaF MgF2 AlF3 SiF4 PF5 SF6
Melting point (oC): 997 1396 1040 -90
-94
-56
Cl2
-101
F2
-220
Account for the trends in the melting points in terms of intermolecular forces
Clearly the ionic compounds (NaCl, MgCl2,AlCl3, BaF, MgF2 and AlF3) have much
higher melting points than the other covalent compounds. This makes sense because
the force holding the ionic solids together is the strong charge/charge interaction, while
the covalent solids are going to be held together by the much weaker dipole/dipole and
London forces.
There are finer points as well. Within the covalent compounds there is a pretty good
range. In particular F2 has the lowest low melting point. This makes sense because is
it nonpolar, so it is held together by London forces, and the London force is proportional
to the size of the compound, so it makes sense that the smallest compound would have
the weakest intramolecular interaction and the lowest melting point. I was hoping that
somebody would recognize PCl3 and SCl2 as polar compounds, and as such should
have the highest MP of the covalents. This is curious because, as the table shows they
are not all that high, so I suspect the other large nonpolars have enough london force
holding them together to put them at the same level as the polars.
3
Finally there is AlCl3 that has a much lower melting point than the other ionic
compounds, so I suspect that this compound is not completely ionic, but is has some
covalent character so it has a melting point that is lower.
5. (12 points) In Chapter 11 I introduced the term )H solution and divided it into three
individual terms. What is )H solution, what were the individual terms, and how are these
terms important in explaining the formation of a solution?
)Hsoln refers to the heat energy that is taken in or given off when a solute is dissolved in
a solvent. In class we subdivided )Hsoln into three separate components:
)H1 the energy required to separate the solute molecules from each other.
)H2 the energy required to open ‘holes’ in the solvent structure that you can
place solute molecules into.
)H3 the energy that can be either gained or lost as the solute and solvent
molecules interact with each other.
The overall sum of these interactions determine if a solution will form. If )Hsoln is
either negative or slightly positive a solution will form. If )Hsoln is large and positive, then
a solution will not form, and the solute and solvent will separate.
6. A (10 points) Liquid A has a vapor pressure of 0.1 atm at room temperature. Liquid
B has a vapor pressure of .05 atm at room temperature. Assuming an ideal solution,
what is the vapor pressure of a solution that contains 3 molecules of A for every 1
molecule of B?
VPsoln = P1 VPpure 1 + P2 VPpure 2
PA=# moles A/total # of moles = 3/4 = .75
PB=# mole B/total # of moles = 1/4 = .25
VP = .75(.1) + .25(.05) = .0875 atm
B (3 points) In reality A and B don’t mix very well and tend to separate out if you add
any more B to the solution. Given this fact, do you think the actual vapor pressure of
the solution will be higher or lower than your calculation? Why?
If they separate, this indicates that )Hsoln is positive, so the molecules don’t want
to associate in the liquid phase, so they will move into the vapor phase to avoid the
liquid phase, hence the pressure till be higher than expected (a + deviation from
Raoult’s law)
4
7. To make candy you dissolve sugar (sucrose, C12H22O11) and some other materials in
water, and then boil the solution for a long time to get rid of the excess water. As the
water is boiled away the sugar becomes more concentrated, and the boiling point of the
solution gets higher and higher.
A. (10 points) Assume you have cooked a candy recipe until it has reached the ‘Soft
Crack’ stage where the boiling point of the solution is 270oF or 132oC. What is the molal
concentration of sucrose in this solution? (The molal boiling point elevation constant for
water is .51 oC"kg/mol)
)T= Kb m
)T=132-100 = 32oC; X=molarity
32oC = 0.51 oC"kg/mol × X
X= 32oC/ 0.51 oC"kg/mol
62.7 mol/kg
B.(3 points) With a boiling point elevated by 32o C you would expect the freezing point
of this solution would be depressed and be much less than 0o C. Yet when you cool the
solution, it forms a solid candy at room temperature. Why?
As the water is boiled off, mostly sugar remains. Since sugar is a solid at room
temperature, the remaining material will be a solid.
8. (12 points) What is the osmotic pressure of a solution that is .1 M NaCl when T=25oC
?
Ordinarily B=MRT
But here you should recognize that the NaCl is ionic, and will ionize with i=2
Thus you have to include a van’t Hoff factor
B= iMRT
=2(.1).08206 l"atm/K"mol ×298.15K
= 4.89 atm