Key to Review Worksheet for Exam 2
Chem 101, Summer 2006
1. Balance the following chemical equations:
+

2ZnO
3CaCl2

a)
2ZnS
b)
2Na3PO4
+
c)
XeF6
3H2O
+
3O2

+
2SO2
Ca3(PO4)2
XeO3
+
+
6NaCl
6HF
2. Identify the type (or types) of reaction for each of the following:
a)
PbO(s) + C(s)  Pb(s) + CO(g)
single replacement, redox
b)
2K(s) + Cl2(g)  2KCl(s)
combination, redox
c)
2AgBr(s)  2Ag(s) + Br2(l)
decomposition, redox
d)
2C6H6(l) + 15O2(g)  12CO2(g) + 6H2O(l) + heat
combustion, exothermic
e)
AgNO3(aq) + H2S(aq)  Ag2S(s) + HNO3(aq)
double replacement, redox
3. For each of the following re-dox reactions, identify what is being oxidized and what
is being reduced:
a) 2Al + 3I2  2AlI3
Oxidized __Al___
Reduced ___I___
b) 2KBr + Cl2  2KCl + Br2
Oxidized __Br___
Reduced __Cl___
c) 2Ag + 2HNO3  2AgNO3 + H2
___H___
Oxidized __Ag___
Reduced
4. Complete and balance the following chemical equations:
a)
2K(s)
+
b)
H2CO3(aq)
c)
CuCl2(aq)
d)
C3H8(g)
e)
Ca(s)

2KBr(s)

CO2(g)
+
+
+
+
Br2(l)
Na2S(aq)
H2O(l)

CuS(s)
5O2(g)

4H2O(l)
2HCl(aq)

CaCl2(aq)
+
+
2NaCl(aq)
3CO2(g)
+
+
heat
H2(g)
5. Draw an energy diagram for a chemical reaction that has the following
characteristics: exothermic and fast reaction. Label the axes, reactants, transition state,
products, activation energy and heat of reaction on your diagram.
Transition State
EA
Energy
Reactants
Heat of Reaction
Products
Reaction Progress
6. How would each of the following change the rate of the following reaction?
2Mg + O2  2MgO + Heat
a) The reaction is cooled to 0C
Faster
Slower
b) The reaction is done in an atmosphere of pure O2 instead of in air (air is a mixture of
gases that include O2)
Faster
Slower
7. Which way will the equilibrium of the following reversible reaction be shifted under
each of the following conditions:
N2 + O2 + heat  2NO
a) Heat is added to the reaction
Towards Reactants
Towards Products
b) NO is added to the reaction
Towards Reactants
Towards Products
c) O2 is added to the reaction
Towards Reactants
Towards Products
8. Consider the compound Mg(OH)2
a) How many atoms of oxygen are in 2.0 moles of Mg(OH)2?
mol Mg(OH)2  mol O  atoms O
2.0 mol Mg(OH)2 x (2 mol O/1 mol Mg(OH)2) x (6.02 x 1023 atoms O/1 mol O)
= 2.4 x 1024 atoms O
b) How many moles of Mg(OH)2 contain 1.5 x 1023 atoms of Mg?
atoms Mg  mol Mg  mol Mg(OH)2
1.5 x 1023 atoms Mg x (1 mol/6.02 x 1023 atoms) x (1 mol/1 mol) = 0.25 mol Mg(OH)2
c) How many grams of hydrogen are in 5.0 g of Mg(OH)2?
g Mg(OH)2  mol Mg(OH)2  mol H  g H
5.0 g Mg(OH)2 x (1 mol Mg(OH)2/58.33 g Mg(OH)2) x (2 mol H/1 mol Mg(OH)2) x
(1.008 g H/1 mol H) = 0.17 g H
9. Consider the following reaction:
2Al + Fe2O3  Al2O3 + 2Fe + 204 kcal
a) What type of reaction is this (besides redox)?
Combination Decomposition
Replacement
Combustion
b) Identify what in the reaction is oxidized and what is reduced:
Oxidized __Al___
Reduced __Fe___
c) Is the reaction
exothermic
or
endothermic
?
d) How many grams of Fe2O3 are required to react with 5.00 g of Al?
g Al  mol Al  mol Fe2O3  g Fe2O3
5.00 g Al x (1 mol/26.98 g) = 0.1853 mol Al
0.1853 mol Al x (1 mol Fe2O3/2 mol Al) = 0.9266 mol Fe2O3
0.9266 mol Fe2O3 x (159.7 g/1 mol) = 148 g Fe2O3
e) If 1.0 mol Al is reacted with 1.0 mol Fe2O3, which is the limiting reactant? (Circle
your answer)
Al
or
Fe2O3
10. Calculate the % composition by mass for Na2S
Molar mass of Na2S = 78.04 g
% Na = (45.98 g/78.04 g) x 100% = 58.92% Na
% S = (32.06 g/78.04 g) x 100% = 41.08% S
(58.92% + 41.08% = 100.00%)
11. Consider the following reaction:
2H2 + O2  2H2O
a) If 10.0 g of H2 is reacted with 10.0 g of O2, which is the limiting reactant?
g H2  mol H2  mol H2O
g O2  mol O2  mol H2O
Choose the reactant that gives the lower yield:
10.0 g H2 x (1 mol/2.016 g) x (2 mol H2O/2 mol H2) = 4.96 mol H2O
10.0 g O2 x (1 mol/32.00 g) x (2 mol H2O/1 mol O2) = 0.625 mol H2O
b) What is the theoretical yield?
g O2  mol O2  mol H2O  g H2O
10.0 g O2 x (1 mol/32.00 g) x (2 mol H2O/1 mol O2) = 0.625 mol H2O (From 15a)
0.625 mol H2O x (18.016 g/1 mol) = 11.3 g H2O
O2
c) If 10.0 g of H2O is produced, what is the % yield?
(10.0 g/11.3 g) x 100% = 88.5%
12. Consider the following reaction:
PCl5  PCl3 + Cl2
a) Write the equilibrium expression:
Keq = [PCl3][Cl2]/[PCl5]
b) If Keq = 2 x 10-2, does the equilibrium favor
reactants
(or)
products
(or)
neither ?
c) If Cl2 is removed from the reaction, does the Keq get
larger
(or)
smaller
(or)
neither ?
13. Complete the following statements regarding gases:
a) At constant T and n, the volume
b) At STP, the volume
increases
c) At constant V and n, the temperature
d) At constant P and n, the density
14.
increases
when P is decreased.
when n is increased.
increases
increases
when P is increased.
when T is decreased.
A steel cylinder with a volume of 25.0 L is filled with 125 g of O2 gas at 20.0C
(R = 0.0821 L atm/mol K).
a) What is the pressure (in atm) of O2 gas in the cylinder?
PV = nRT
P = nRT/V
n = 125 g O2 x 1 mol/32.00 g = 3.906 mol
P = (3.906 mol)(0.0821 L x atm/mol x K)(293 K)/25.0 L = 3.76 atm
b) If 500.0 g of N2 gas is added to the cylinder, what is the total pressure in the
cylinder?
n = 500.0g x 1 mol/28.02 g = 17.84 mol
P = nRT/V = (17.84 mol)(0.0821 L x atm/mol x K)(293 K)/25.0 L = 17.17 atm
Ptotal = P1 + P2 = 3.76 atm + 17.17 atm = 20.9 atm
15. What is the density (in g/L) of H2S gas at STP?
1 mol H2S = 34.016 g = 22.4 L
d = m/V = 34.016 g/22.4 L = 1.52 g/L
16. A balloon is filled with He gas to a volume of 1.0 L. If the pressure and the moles
of He are held constant, and the temperature goes from 20.0C to 40.0C, what is the
new volume?
P1V1/T1 = P2V2/T2
Pressure is constant: P1 = P2 V1/T1 = V2/T2
V1 = 1.0 L
T1 = 20.0C + 273 = 293 K
T2 = 40.0C + 273 = 313 K
V2 = V1T2/T1 = (1.0 L x 313 K)/293 K = 1.1 L
17. Consider the following reaction:
2H2(g) + O2(g)  2H2O(g)
a) How many liters of H2 gas are required to react with 1.0 L of O2 gas at STP?
L O2  mol O2  mol H2  L H2
1.0 L O2 x (1 mol/22.4 L) x (2 mol H2/1 mol O2) x (22.4 L/1 mol) = 2.0 L
b) How many liters of H2O gas are produced when 10.0 g of H2 gas are reacted with
sufficient O2 at STP?
g H2  mol H2  mol H2O  L H2O
10.0 g H2 x (1 mol/2.016 g) x (2 mol H2O/2 mol H2) x (22.4 L/1 mol) = 111 L H2O
18. Indicate whether each of the following substances are soluble in water:
a) CaOH
No
b) K2S
Yes
c) H2S
Yes
d) Ba(NO3)2
Yes
e) PbSO4
Yes
f) C8H18
No
19. Write a balanced equation for the dissociation of the following electrolytes in water:
a) NaNO3 (strong)
NaNO3  Na+ + NO3-
b) Cu(NO3)2 (strong)
Cu(NO3)2  Cu2+ + 2NO3-
c) HF (weak)
HF + H2O  F- + H3O+
20. KCl has a solubility of 42.6 g/100 g H2O at 50C. How many grams of KCl are
required to make a saturated solution with 50.0 mL of H2O at 50.0C? (density of H2O
= 1.00 g/mL)
mL H2O  g H2O  g KCl
50.0 mL H2O x (1.00 g/mL) x (42.6 g KCl/100 g H2O) = 21.3 g KCl
21. KCl has a solubility of 42.6 g/100 g H2O at 50C and 34.0 g/100 g H2O at 20C. If a
saturated solution of KCl is made with 50.0 mL of H2O at 50C and then cooled to 20C,
how many grams of KCl will precipitate out of the solution? (density of H 2O = 1.00
g/mL)
g KCl dissolved at 50C = 21.3 g (see above, question 3)
g KCl dissolved at 20C = 50.0 mL H2O x (1.00 g/mL) x (34.0 g KCl/100 g H2O) = 17.0 g
g KCl that precipitate = 21.3 g – 17.0 g = 4.3 g
22. If ale is 7.5% (v/v) alcohol, how many ounces of alcohol will you consume if you
drink 4.0 pints? (1 pint = 16 oz)
pints ale  oz ale  oz alcohol
4.0 pints x (16 oz/1 pint) x (7.5 oz alcohol/100 oz ale) = 4.8 oz alcohol
23. If you were in the laboratory, how would you prepare each of the following
solutions?
a) 1.0 L of 5.0% (m/v) glucose
L solution  mL solution  g glucose
1.0 L x (1000 mL/1 L) x (5.0 g glucose/100 mL) = 50. g glucose
Weigh out 50. g glucose and place it in a 1.0 L volumetric flask. Add some water to
dissolve glucose, then add water to 1.0 L total volume.
b) 5.00 L of 0.100 M KCl
5.00 L x (0.100 mol KCl/1 L) x (74.55 g/1 mol) = 37.3 g KCl
Weigh out 37.3 g KCl and place it in a 5.00 L volumetric flask. Add some water to
dissolve KCl, then add water to 5.00 L total volume.
24. Consider the following reaction:
Na2CO3(aq) + 2HCl(aq)  CO2(g) + H2O(l) + 2NaCl(aq)
How many mL of a 2.00 M HCl solution are required to react with 3.00 g of Na2CO3?
3.00 g Na2CO3 x (1 mol/105.99 g) = 0.0283 mol Na2CO3
0.0283 mol Na2CO3 x (2 mol HCl/1 mol Na2CO3) = 0.0566 mol HCl
0.0566 mol HCl x (1 L/2.00 mol) x (1000 mL/1 L) = 28.3 mL HCl solution
25. State whether each of the following substances would form a solution, colloid or
suspension when mixed with water:
a) starch
colloid
b) NaCl
solution
c) oil
suspension
d) glucose
solution
26. Equal volumes of two solutions, A and B, are placed into a beaker where they are
separated by a semi-permeable membrane that allows only water to pass through.
A = 1.0 M NaCl and B = 1.0 M CaCl2.
a) Which solution has the higher osmotic pressure?
B (CaCl2 has more particles per mole than NaCl)
b) On which side (A or B) will the volume increase over time?
B (water flows from A to B due to osmosis)
27. What happens to a red blood cell when it is placed into each of the following:
a) sea water
It shrivels (water goes out of the cell due to osmosis).
b) distilled water
It bursts (water goes into the cell due to osmosis).
28. If a mixture of protein, starch, glucose and NaCl in water is placed into a dialysis
bag, and the bag is put into a beaker of distilled water, which components will flow out
of the bag?
Glucose and NaCl will flow out of the bag.