Name:_____________
(2 points)
Chemistry 114
Second Hour Exam
Remember- Show all work for partial credit.
All problems worth 14 points. You only have to answer 7 of the 8 questions!
If you attempt all 8 I will give you your 7 best scores.
1.Classify each of the following crystalline solids, and tell what forces are important in
holding the crystal lattice together
Type of crystalline solid
Force
F2
Molecular_______
London_________
Cr
Atomic-metallic
Delocalized Covalent
Cgrahite
Atomic-network
Directional Covalent
MgCl2
Ionic_____________
Charge-charge
Ne
Atomic - group 8A
London_________
Si
Atomic-network
Directional covalent
H2S
Molecular_______
Dipole-dipole_________
2. If the distance between 2 atoms in silver is 344 pm. When silver is examined with a
2.62 Å wavelength X-ray, at what angle would you expect to see a first order (n=1)
diffraction? (1Å = 10-10m, 1 pm =10-12 m)
2
3.
A. Material AxBy forms a crystal structure in which the A atoms form a simple
cubic lattice, and the B atoms occupy the body center of half the cubes. What is the
molecular formula for this compound.
For 1 cube A = 8 x 1/8=1
For 2 cubes A =2
For 2 cubes B =1
A2 B1
B. Material AxBy forms a crystal structure in which the A atoms form a simple
cubic lattice, and the B atoms occupy the centers of each edge. What is the molecular
formula for this compound.
A = 8 x 1/8 =1
B = 12 x1/4 =3
A1 B3
C. Material AxBy forms a crystal structure in which the A atoms form a simple
cubic lattice, and the B atoms occupy the face centers the cubes. What is the molecular
formula for this compound.
A = 8 x 1/8 =1
B = 6 x ½ =3
A1 B3
4. Ethanol has a vapor pressure of 45 torr @ 20 oC, and a normal boiling point of
78 oC. What is the ÄH vaporization of ethanol in kJ?
3
5. Define the following terms. In some cases you may give equations as definitions
mole fraction
na/(na + nb)
Henry’s law
c=KP
non-ideal solution
A solution that does not follow Raoult’s law.
osmosis
A solute moving from a region of high concentration to a region of low
concentration.
boiling point elevation
ÄT=KB m
6. Remembering that solids can be solutions too, try this on for size....
Brass is a uniform mixture of zinc and copper. If 1000 cm3 of brass weighs 8.4
kg and is 25% by weight zinc.
A. (2 points) How many grams of zinc are there in 1 liter of brass?
8.4 kg x 25/100 = 2.1 kg
B. (2 points) How many grams of copper are there in 1 liter of brass?
8.4 kg - 2.1 kg = 6.3 kg
C. (2 points) What is the molarity of zinc in brass?
2,100 g / 65.38 g/mol = 32.1 mol
32.1 mol/1l = 32.1 M
D. (3 points) What is the molality of zinc in brass?
32.1 mol Zn / 6.3 kg Cu = 5.1 m Zn
E. (3 points) What is the mole fraction of zinc in brass?
Mole Cu = 6,300 g Cu / 63.55 g/mole Cu = 99.13 mol Cu
÷=32.1/(32.1+99.1) = .24÷Zn
4
7. I am going to mix 50 g of ethanol (molar mass 46.1) with 50 of acetone (molar mass
58.1) If the vapor pressures of these two liquids are 45 torr and 183 torr, respectively
what should the vapor pressure of the mixture be if they form an ideal solution?
Mole Ethanol = 50 g / (46.1g/mol) = 1.087 mol
Mole Acetone = 50 g/(58.1 g/mol) = .861 mol
VPSolution =VPEthanol ÷Ethanol + VPAcetonet ÷Acetone
=45(1.087/(1.087+.861)) + 183(.861/(1.087+.861))
=106.0 torr
Would the vapor pressure of the solution get higher or lower if the solution got colder
upon mixing? Why?
If the solution got colder it had a positive ÄHSolution indicating that the solvent and
solute have poor interactions with each other. When that happens the VP of the
solution will typically become higher (+ deviation from Raoult’s law) because the
molecules will try to escape into the vapor to avoid the solution.
8. When you make salt water taffy you add enough sugar, corn syrup and butter to
water to raise the boiling point from 122 oC. What is the molality of the solution at this
point? (The boiling point elevation constant of water is 0.51 oC@kg/mol)
ÄT = KB m
ÄT = 122-100 = 22
220 = 0.510CAkg/mol × X
220/0.510CAkg/mol = X
X = 43.1mol/kg