Name:_____________
(4 points)
Chemistry 114
Second Hour Exam
Remember- Show all work for partial credit. Each problem is worth 12 points.
1. 65 grams of MgCl2 is dissolved in 300 g of water. Assuming that water has a density
of 1.00 g/mL
A. What is mole fraction of MgCl2 in water?
Molar mass MgCl2 = 24.31 + 2(35.45) = 95.21; Moles = 65/95.21 = .683
Molar mass H2O = 16.00 + 2(1.008) = 18.016; Moles = 300/18.016 = 16.65
÷=.683/(.683+16.65) = .0394
B. What is the molality of MgCl2 in this solution?
300 g H2O ×1 kg/1000g = .300 kg
m=moles solute/kg solvent = .683/.300 kg = 2.28 m
C. What is the colligative molality of this solution?
mc = im; MgCl2 6Mg2+(aq) + 2Cl-(aq) ; i=3
mc = 3(2.28) = 6.83 m
2. Normal BP of cyclohexane is 80.7 oC The Boiling Point elevation constant of
cyclohexane is 2.92 kg/mol@oC. If solution of cyclohexane has a BP of 85oC...
A. What is the molality of the solution?
ÄT = KB @m
ÄT = 85.0-80.7 = 4.3o
4.3o = 2.92 kg/mol @ oC × m
m = 4.3oC / 2.92 kg/mol@oC
= 1.47 mol/kg
B. If the solution was made with 40g of cyclohexane how many moles of solute
are in the solution.
1.47 mol solute/kg solvent = X mol/0.040 kg
X = 1.47mol/kg solvent × .040 kg solvent
= .0589 mole solute
C. If the solution was made with 1.0 g of solute, what is the molar mass of the
solute.
Molar mass = g/mol
= 1.0g/.0589 mol
= 16.98 g/mol
Actually a very low molar mass!
2
3. Assuming equal concentrations, rank these aqueous solutions by their osmotic
pressure:
Aluminum chloride, Tin(IV) nitrate, NH4NO3, Na2SO4,
Highest
Lowest
Ð= iMRT
If all the concentrations are the same then the MRT term is the same
so the highest osmotic pressure will come from the compound with the largest I
and the lowest osmotic pressure from the compound with the lowest I
Aluminum Chloride = AlCl3 6Al3+(aq) + 3Cl-(aq),
Tin(IV) nitrate = Sn(NO3)4 6Sn4+(aq) + 4NO3-(aq),
NH4NO3 6 NH4+(aq) + NO3-(aq)
Na2SO4 62Na+(aq) + SO42-(aq)
Tin(IV) nitrate
Aluminum chloride
Na2SO4
i=4
i=5
i=2
I=3
NH4NO3
4. When liquids A and B are mixed together, the vapor pressure of the solution is
higher than what you calculate using Raoult’s law....
A. This indicates a positive or negative deviation from Raoult’s law? (Circle one)
B. This indicate that the two liquids interact well or poorly? (Circle one)
C. Explain:
Higher pressure indicates that molecules in liquid are moving into the vapor phase at a
faster rate because they are not interacting with each other in the liquid phase
D. The structures of the two liquids are probably similar or different? (Circle one)
3
5. Define the following terms:
colligative molality
mc = im Molality adjusted for the fact that ionic solutes can dissociate
van’t Hoff factor
The i in the above equation. It idicates how many particles a solute can become
when it dissolves in the solute.
azeotrope
A solution that distills without a change in composition.
rate law
rate = k[A]y: A mathematical equation that relates the rate of a reaction to a
constant times the concentration of one or more reactants raised to some power.
order parameter
Y in the above equation. The exponent used raise a concentration to a power in
a rate law equation.
colligative property
A property that depends on the number of solute particles dissolved in a solvent,
not the chemistry of the solute molecules.
6. The rate of the following reaction is 0.58 M/s. What is the relative rate of change of
each species in the reaction
A +2B 63C
A. Ä[A]/Ät=
rate = -1/a ÄA/Ät
.58 M/s = -1/1 ÄA/Ät
ÄA/Ät= -.58 M/s
B. Ä[B]/Ät =
rate = -1/b ÄB/Ät
.58 M/s = -1/2 ÄB/Ät
ÄB/Ät= -1.16 M/s
C. Ä[C]/Ät =
rate = 1/c ÄC/Ät
.58 M/s = 1/3 ÄC/Ät
ÄC/Ät= 1.74 M/s
4
7. A6B is a first order reaction with respect to A
If the initial concentration of A is .15 M and the rate of the reaction is .20 min-1, what is
the concentration of A after the reaction has run for 17 minutes?
Ln[A]t = Ln[A]0 - kt
Ln[A]t = Ln(.15) - .20(17)
Ln[A]t= -1.897 - 3.40
Ln[A]t= -5.297
[A]t = e-5.297 = .005 M
8A. Write the equations for:
The half-like of a zero order reaction.
t1/2 = [A]0/2K
The half-life of a first order reaction.
t1/2 = .693/k
The half-life of a second order reaction.
t1/2 = 1/k[A]0
8B. What do you plot (X and Y axes) to tell if a reaction is
Zero order?
X=time, Y = concentration
First order?
X=time, Y = ln(concentration)
Second order?
X=time, Y = 1/ concentration