Name:_____________
(4 points)
Chemistry 114
Second Hour Exam
Remember- Show all work for partial credit.
1. (12 points) I am doing X-ray diffraction of a protein crystal using an X-ray with a
wavelength of 1.54 Å. The unit cell in this crystal has a distance d of 8.1 nm. At what
angle (in degrees) should I expect my first (n=1) reflection from this crystal.
(Note: protein crystals are very large compared to the crystals used as examples in
your text, so you final answer my look a little odd!)
1.54 Å = 1.54×10-10m
8.1 nm = 8.1×10-9m
2. (12 points) For each of the five chemicals below give the type of solid, bonding
between lattice points in the solid, and estimate the melting point of the solid.
Type of solid
Bonding
Melting point
C(diamond) Atomic Network
Directional Covalent
High
Sn
Atomic Metallic
Non-directional Covalent
High
Xe
Atomic Group 8A
London Forces
Low
OF2
Molecular
Dipole Forces
Medium
CaF2
Ionic
Electrostatic Forces
High
Types of solids
Amorphous
Ionic
Molecular
Atomic network
Atomic metallic
Atomic Group 8A
Bonding between lattice points
Non-directional Covalent
Directional Covalent
Dipole forces
London Forces
Electrostatic Forces
Melting point
High
Medium
Low
2
3. (12 points) If water is boiling today in Spearfish at 95.1oC, what is the air pressure?
(Assume the ÄHvap of water is 43.9 kJ.mole)
95.1oC = 368.25K, 100oC = 373.15K; 43.9 kJ = 43,900J
4. (12 points) Substance Z has the following properties:
ÄH vap 20. kJ/mol Specific Heat Capacity of solid = 3.0 J/g@oC
ÄH fus 5.9 kJ/mol
Specific Heat Capacity of liquid = 2.5 J/g@oC
o
bp
75 C
Specific Heat Capacity of gas = 1.0 J/g@oC
mp
-15oC
Molar Mass = 50.0 g/mol
How much energy will it take to change the temperature of 100 g of Substance Z from
25oC to 100oC?
At 25oC substance Z is a liquid you need to raise the temp up to the boiling point (75o C)
Energy1 = ÄT x S.H.C. x g = (75-25)2.5(100) = 12,500J
Z is now at it’s boiling point so it must now be vaporized
Energy2 = ÄHVap x moles Z = 20,000J x (100/50) = 40,000J
Finally you must raise the temp of the gas up to the boiling point (100o C)
Energy3 = ÄT x S.H.C. x g = (100-75)1.0(100) = 2,500J
Energy = E1 + E2 + E3 = 55,000 J = 55 kJ
3
5.One liter of concentrated HCl weighs 1,190 grams and is 37.3% HCl by mass.
A. (2 points) How many grams of HCl are there in 1 liter of concentrated HCl?
1,190 g x .373 = 443.9 g HCl
B. (2 points) How many grams of water are there in 1 liter of concentrated HCl?
1,190g - 443.9 = 746.1 g H2O
C. (2 points) What is the molarity of concentrated HCl?
Moles HCl = 443.9g/36.46g = 12.18 moles in 1 liter = 12.18M
D. (3 points) What is the molality of concentrated HCl?
Molality = moles HCl/kg solute = 12.18moles/.7461kg = 16.25m
E. (3 points) What is the mole fraction of HCl in this solution?
Moles H2O = 748.1/18 = 41.45 moles
mole fraction = 12.18/(12.18+41.45) = .227
6. (8 points) Define the following terms:
ÄH fus:
Heat required to change 1 mole of a solid to a liquid at the material’s melting point.
Molality:
Moles solute/ kg solvent
ÄH solution:
Heat gained or lost as a solute dissolves in a solvent.
Lattice Energy:
Energy released in the reaction M+(g) + X-(g) 6MX(solid)
7. (4 points) Here is a vitamin you didn’t have in class, vitamin K1.
Would this be classed as a fat soluble or a water soluble vitamin, Why?
Compound is non-polar so this would be a fat soluble vitamin.
4
8. (12 points) I have a 591 ml bottle of soda in my office. If this soda was canned
under a 5 atm pressure of CO2, how many liters of belches will it give me? Or, to put it
more scientifically, if the Henry’s Law constant (k) for CO2 is .031 mol/l@atm, how many
moles of CO2 are dissolved in the soda, and what is the volume, in liters, of this gas it
will release at 35oC (body temperature) and .88 atm (Spearfish pressure).
C = kP
C = .031 x 5 atm = .155M
You have 591 ml = .591 liters
.155 moles/liter x .591 liters = .0916 moles CO2
PV = nRT
V = nRT/P
= .0916(.08206)(35+273)/.88
=2.63 liters of CO2 gas
9A. (8 points) Substance A has a vapor pressure of 250 torr at 25oC. Substance B has
a vapor pressure of 110 torr at the same temperature. If I mix 3 moles of A with 2
moles of B, what should the vapor pressure of the resulting solution be?
VP = ÷1VP1 + ÷2VP2
= 3/5(250) + 2/5(110)
= 150 + 44
= 194 torr
9B (4 points) If the actual vapor pressure of this solution is 210 torr did the solution get
warmer or colder when it was mixed? Why?
Actual VP is higher than theoretical so we have a + deviation from Raoult’s Law.
Positive deviations mean that there are more molecules in the vapor phase than there
should be. This is because there are poor solvent-solute interactions. When there are
poor solvent-solute interactions, the solution needs heat (+ÄHsolution) to form so the
solution gets colder as it is mixed.