Chemistry 141 - Problem Session 4
1.
A room in
which the humidity has been lowered feels cooler.
Yet the dehumidifier has not “cooled” the room.
Why is that?
2.
How might
one go about establishing whether a solution of sodium sulfate in water is
saturated, unsaturated, or supersaturated?
3.
Here is
an experiment you can do at home: Slice a raw potato across the bottom so it
will sit flat. Dig a 2-inch hole in
the top side and fill it 2/3 full of sugar.
After a while, the sugar will be moist, perhaps even watery.
Why is that?
4.
Pure
iodine (100.0 g) is dissolved in 300.0 g of CCl4 at 65oC.
Given that the vapor pressure of CCl4 at this temperature is
504 mm Hg, what is the vapor pressure of the CCl4/I2
solution at 65oC? (Assume
that I2 does not contribute to the vapor pressure.)
5.
At 30oC
the vapor pressure of pure acetone (C3H6O) is 285 torr and
that of pure ethyl acetate (C4H8O2) is 118 torr.
What is the vapor pressure of a mixture composed of 15.0 g acetone and
25.0 g of ethyl acetate at 30oC?
(Hint: This has two volatile
solvents so you need to determine the vapor pressure of each.)
6.
A 10.7 m
solution of NaOH has a density of 1.33 g/mL at 20oC.
Calculate
a.
The mole
fraction of NaOH
b.
The
weight percentage of NaOH
c.
The
molarity of the solution
7.
A
solution is 28.0 % by mass ammonium sulfate, with a density of 1.1612 g/mL.
Calculate the molality, molarity and mole fraction of ammonium sulfate in
the solution.
8.
A
solution of glucose (C6H12O6) is prepared by
dissolving 100.0 g of glucose in 1000. g of water. The density of the resultant solution is 1.050 g/mL.
Kb for water is 0.52 oC/m and kf for water is –1.86 oC/m.
a.
What is
the vapor pressure of the solution at 100.0oC?
b.
What is
the boiling point of the solution?
c.
What is
the osmotic pressure of the solution at 25oC?
9.
A
compound of carbon, hydrogen, and oxygen was burned in oxygen, and 1.000 g of
the compound produced 1.434 g CO2 and 0.783 g H2O.
In another experiment, 0.1107 g of the compound was dissolved in 25.0 g
of water. This solution had a
freezing point of -0.0894oC. What
is the molecular formula of the compound?
10.
The
osmotic pressure of blood at 37oC is 7.7 atm.
A solution that is given intravenously must have the same osmotic
pressure as the blood. What should
be the molarity of a glucose solution to give an osmotic pressure of 7.7 atm at
37oC?
11.
For the equilibrium
C(s) +
2 H2(g) <==>
CH4(g) + heat
a.
Write the equilibrium constant expression as Kc.
b.
What are the units of the equilibrium constant Kp?
c.
How is Kc related to Kp (specify quantitatively)
d.
For each of the following changes to the system at equilibrium, predict
the direction of the shift and explain why it occurs:
|
Change
|
Shift
(left or right) |
Reason |
|
The volume of the reaction vessel is doubled. |
|
|
|
The temperature is increased. |
|
|
|
The pressure of H2(g) is increased. |
|
|
|
C(s) is added to the system. |
|
|
|
Adding a catalyst |
|
|
12.
The equilibrium constant for the reaction N2O4(g)
<==> 2 NO2(g)
is 0.212 mol/L at 100o C.
What is the value of Kc at 100oC for:
a.
2 NO2(g) <==>
N2O4(g)
b.
NO2(g) <==>
1/2 N2O4(g)
13.
For the reaction NO(g) +
NO2(g) + H2O(g)
<==> 2 HNO2(g),
occurring at 28oC, [NO]i =
[NO2]i = 44.1 torr and [H2O]i = 17.5 torr. If the
total pressure at equilibrium is 95.6 torr.
a.
What are the equilibrium pressures of all species?
P(NO)
=
P(NO2)
=
P(H2O)
=
P(HNO2)
=
b.
Calculate Kp for the reaction.
14.
At some temperature the system 2
SO2(g) +
O2(g) <==>
2 SO3(g) is at
equilibrium when [SO2]
= 0.0100 M, [O2] = 0.200 M and [SO3] = 0.100 M.
What is the value of Kc at this temperature?
Challenge If at the same temperature, 3.00 mol of SO3
is added to a 1.00 L vessel, what will be the value of [O2] at
equilibrium? (hint: check your
answer – iterate to within 5%)
15.
The reaction 2 NO(g)
+ Br2(g)
<==> 2 NOBr(g)
has a Kp = 1.17 atm-1
at 25oC. If 1.10 atm of
NOBr, 0.100 atm of NO, and 0.0100 atm of Br2 are mixed at 25oC,
what reaction will occur? Explain.
a.
When 5.00 atm of NOBr is allowed to equilibrate at 50oC, the
equilibrium pressure of NOBr is measured to be 4.30 atm.
What is the value of Kp at 50oC? Compare with the value of Kp at 25oC and explain.
16.
The reaction A + 2 B
<==> C
+ D has a Keq
of 4.00 x 10-16
M-1
at 25oC. If 0.500 mol of
C are added to 1.00 mol of D in a 2.00 L vessel and allowed to equilibrate, what
will be the molar concentration of each species at equilibrium?
17.
Solution A is prepared by adding 0.3171 g of KOH to enough water to yield
2.000 L of solution. Solution B is
prepared by dissolving 0.2817 g of HBr in 1.000 L of solution. Fill in the chart for solutions A, B, and C.
[H+]
[OH-]
pH
pOH
Solution
A
Solution
B
Solution
C
(15.0
mL A + 20 ml B)
18.
Calculate the pH of the following solution and the percent dissociation
of 1.00 M HA, a weak acid for which Ka = 4.40 x 10-4M
19.
For each of the following ionic compounds, identify the conjugate acid of
the anion. Where appropriate,
identify the conjugate base of the cation.
Determine which of the following salts would produce acidic, basic, or
neutral solutions. For those
solutions that are not neutral, write the equations to explain the acidity or
basicity.
a.
NaClO4
b.
KF
c.
KC2H3O2
d.
NH4Br
20.
Saccharin,
a sugar substitute, is a weak acid with pKa = 2.32 at 25oC.
It ionizes in aqueous solution as follows:
HNC7H4SO3(aq)
+ H2O(l)ßà H3O+(aq)
+ NC7H4SO3–(aq)
What
is the pH of a 0.10 M solution of this substance?
21.
A 0.150 M
solution of H2Z exhibits a pH of 9.85.
a.
Write an
equation for the reaction of H2Z with water.
b.
Calculate
the value of the equilibrium constant for the reaction (Ka or Kb)
c.
Calculate
the percent dissociation/hydrolysis of 0.150 M H2Z
d.
For a
1.00 M solution of H2Z, calculate the values of [H+], [OH–],
pH, and pOH.
22. .Assume
that you place a fresh water plant into a salt solution and examine it under
a microscope. What happens
to the plant cells? What if you
placed a salt-water plant in pure water? Explain.
Draw pictures to illustrate your explanation.