Chemistry 114
Second Hour Exam
Name:____________
1. (12 points) Which of the following 3 compounds would have the highest boiling point?
The lowest melting point?
Most polar
Highest B.P
Highest M.P.
Least polar
Lowest B.P
Lowest M.P.
2. (13 points) I have an atomic solid which occupies a cubic closest packed array. The
distance between atoms in this array is 3.5Å. If I direct 1.54Å X-rays at this substance, at
what angle will the X-rays be reflected off the crystal surface . (Assume a first order
reflection where n=1.)
n8=2dsin2
1(1.54Å) = 2(3.5Å)sin2
1.54Å/2(3.5Å) = sin2
.220=sin2
2=sin-1(.220)
2=12.7o
2
3. (12 points) A ‘superalloy’ of nickel and aluminum has been described. In this alloy the
metals occupy a lattice in which the Al atoms are at the corners of a cubic unit cell, while
the Ni atoms are at the face centers of the unit cell. What is the composition of this alloy?
(How many Ni atoms are there for every Al atom?) Would this be a substitutional or an
interstitial alloy?
Aluminum in corners, 1/8 x 8 = 1 Al
Ni on faces, ½ x 6 =3 Ni
Ni:Al = 3:1
If you assumed the original Al lattice was a face centered cubic, and the Ni simply
substituted in, then this is a substitutional alloy. If you assumed that the Al lattice was a
simple cubic and the Ni is in a ‘hole’ between the 4 Al’s, then you can argue this as being
an interstitial.
5. (13 points) Create a phase diagram for a substance that is consistent with the following
data:
1. The material has a normal melting point of 45oC and a normal boiling point of 135o.
2. At 200 Atm the material has a solid-fluid phase transition at 100oC, but no liquid/gas
transition is ever seen at this pressure.
3. The material has a solid-gas transition at 0.001 atm and -50oC.
3
6 A. (5 points) I am going to mix 50g of acetone (molar mass 58.1) with 25 g of water
(Molar Mass 18). What is the mole fraction of acetone in this mixture?
6B (5 points) At 25oC the vapor pressure of acetone is 345 torr, and the vapor pressure of
water is 24 torr. If the water acetone solution you made in 6A is an ideal solution, what
should the vapor pressure of this mixture be?
V.P. = Pacetone V.Poacetone + PwaterV.P.owater
= .382(345)+.618(24) (Owater=1-.382)
= 146.6 torr
6C (5 points) If the actual vapor pressure of the solution is 300 torr, is the solution ideal or
does it deviate from Raoult’s law. If it does deviate, is it a positive deviation or a negative
deviation,
Actual > theoretical; This is a + deviation
6D (5 points) Based on your answer in 6C, would you expect water and acetone to get
warmer, colder, or remain at the same temperature when you mixed them together?
+ Deviation associated with + )H solution which means the solution gets colder
when you mix it up
4
7 A. (10 points) I want to make the ice on my sidewalk melt when it is -100C (+14oF)
outside. If the Kf (molal freezing point depression constant) for water is 1.86 oC@kg/mol,
what is the molality of NaCl that I will have to achieve to make the ice melt?
I want a )T of 10oC
)T=Kfim (NaCl is ionic and makes 2 ions, so i=2)
10 = 1.86(2)m
m = 10/1.86(2)
= 2.69 mole/kg solvent
7B (10 points) Lets assume I have about 4 liters of ice (roughly a gallon) that I have to melt,
how much NaCl will I need to use? (Molecular mass of NaCl is 58.44 g/mol)
4 liters is 4000 ml, and at 1 g/ml, that is 4000g or 4 kg of water
m = 2.69 moles/kg solvent x 4 kg solvent
= 10.76 moles NaCl
10.76 moles x 58.44 g/mole = 628.8g NaCl
7C (10 points) I ran out of salt so I rummaged through the chemical stockroom to find
something else to use. I can up with three different compounds that sounded like they
might not be too harmful: Glucose (C6H12O6), Magnesium chloride (MgCl2) and Cesium
Iodide (CsI). Gram-for-gram which of these compounds would be the most effective at
melting my ice (Which will take the least number of grams)
There are 2 problems here, molecular weight, and the van’t Hoff factor. What I am
looking for is the biggest )T/gram material, so the easiest approach is to simply calculate
the )T for 1 gram of each chemical in 1 kg of water.
Compound
Glucose
I=
1
MW = (6x12)+12 + (6x16) = 180
)T = KBim
=1.86(1)(1/180)
=.01
MgCl2
3
24.3+2(35.45)=95.2
CsI
2
132.9 + 126.9=259.8
=1.86(3)(1/95.2)
=1.86(2)(1/259.8)
.06
.014
This is the best
You actually didn’t have to do a calculation, the combination of largest i and lowest MW
makes MgCl2 the winner without doing any calculations