Department of Chemistry, Geosciences, and Environmental Science
Chemistry 1054-010 College Chemistry Summer I 2005
Exam 2-Key
Name:_______________________________________________________
(Please write your name legibly)
Read all directions and questions carefully!! This exam consists of two parts. The first part consists of 10 multiple choice questions worth four points each for a total of 40 points. The second part consists of five numerical problems worth either 10 or 15 points per question for a total of 60 points. Show all your work necessary for the numerical problems as partial credit will be given for those problems.
Possibly Useful Constants
Avogadro’s Number: NA = 6.022 × 1023 particles/mol
Score
Part 1 (40 points):_____________________
Part 2 (60 points):_____________________
Total (100 points):_____________________
Don’t forget to put your name on this test!
Good Luck!!
Part 1
Multiple Choice
Please indicate the answer to each question by putting your choice in the space provided. There is only one correct answer for each question. There will be 10 multiple choice questions worth 4 points each.
1. A strong electrolyte is one that ______________ completely in solution.
(a) reacts (c) disappears
(b) decomposes (d) ionizes
(e) oxidizes
Answer: D
2. A neutralization reaction between an acid and a metal hydroxide produces
(a) water and a salt (c) hydrogen gas
(b) oxygen gas (d) sodium hydroxide
(e) ammonia
Answer: A
3. Lithium and oxygen react to produce lithium oxide according to the reaction:
4 Li(s) + O2(g) ¾¾® 2 Li2O(s)
Suppose 4.0 mol of Li and 2.0 mol of O2 are reacted. What is the limiting reactant and what is the theoretical yield of Li2O for this reaction?
(a) Li, 60. g of Li2O (c) O2, 120 g of Li2O
(b) Li, 30. g of Li2O (d) O2, 60. g of Li2O
Answer: A
4. What is the concentration of sulfate ions in 0.30 M Al2(SO4)3?
(a) 0.10 M (c) 0.30 M
(b) 0.15 M (d) 0.60 M
(e) 0.90 M
Answer: E
5. Which one of the following compounds is insoluble in water according to the solubility rules?
(a) Na2CO3 (c) K2SO4
(b) Fe(NO3)3 (d) ZnS
(e) AgNO3
Answer: D
6. The DE for a system that performs 213 kJ of work on its surroundings and loses 79 kJ of heat is equal to:
(a) +292 kJ (c) +134 kJ
(b) –292 kJ (d) –134 kJ
(e) –213 kJ
Answer: B
Standard Enthalpy of Formation Values
Substance |
DHf° (kJ/mol) |
Substance |
DHf° (kJ/mol) |
Substance |
DHf° (kJ/mol) |
H2O(l) |
–286 |
NO(g) |
90 |
NH3(g) |
–46 |
HNO3(aq) |
–207 |
NO2(g) |
34 |
|
|
7. Given the data in the table above, calculate DH° for the reaction:
3 NO2(g) + H2O(l) ¾¾® 2 HNO3(aq) + NO(g)
(a) 64 kJ (c) –140 kJ
(b) 140 kJ (d) –508 kJ
(e) –64 kJ
Answer: C
8. A chemical reaction that absorbs heat is said to be _______________ and has a ______________ value of DH at constant pressure.
(a) endothermic, positive (c) exothermic, positive
(b) endothermic, negative (d) exothermic, negative
(e) exothermic, neutral
Answer: A
9. Of the substances below, which one has the highest fuel value?
(a) natural gas (c) gasoline
(b) wood (d) hydrogen
(e) coal
Answer: D
Cu(s) + Zn(NO3)2(aq) ¾¾® N.R.
Cu(s) + Ni(NO3)2(aq) ¾¾® N.R.
Cu(s) + HCl(aq) ¾¾® N. R.
Zn(s) + Ni(NO3)2(aq) ¾¾® Zn(NO3)2(aq) + Ni(s)
Zn(s) + 2 HCl(aq) ¾¾® ZnCl2(aq) + H2(g)
Zn(s) + Cu(NO3)2(aq) ¾¾® Zn(NO3)2(aq) + Cu(s)
Ni(s) + Zn(NO3)2(aq) ¾¾® N.R. (no reaction)
Ni(s) + 2 HCl(aq) ¾¾® NiCl2(aq) + H2(g)
Ni(s) + Cu(NO3)2(aq) ¾¾® Ni(NO3)2(aq) + Cu(s)
10. Given the set of reactions shown above, which of the following choices have the species in order of decreasing activity (highest activity on the left, lowest activity on the right?
(a) Copper, Hydrogen, Nickel, Zinc (c) Zinc, Nickel, Hydrogen, Copper
(b) Zinc, Nickel, Copper, Hydrogen (d) Hydrogen, Copper, Nickel, Zinc
(e) Nickel, Zinc, Hydrogen, Copper
Answer: C
Part 2
Numerical Problems
Solve the following problems, keeping track of significant figures where applicable. Please show all the work necessary to obtain your answer in order to receive partial credit for possibly wrong answers. Generally, full credit will not be given for the correct answer without any of the work performed to obtain the answer being shown on the paper. Each question is worth either 10 or 15 points.
11. (10 points) How many grams of K2Cr2O7 are present in 50.0 mL of 0.850 M K2Cr2O7?
Convert the volume into liters:
Multiply the molarity by the volume in L to calculate the number of moles of K2Cr2O7:
Multiply by the number of moles by the molar mass of K2Cr2O7 to calculate the number of grams:
12. (15 points) Disulfur dichloride, S2Cl2, is used in the vulcanization of rubber, a process that hardens natural rubber. It is prepared by heating elemental sulfur, S8, in the presence of chlorine gas:
S8(l) + 4 Cl2(g) ¾¾® 4 S2Cl2(l)
___ What is the maximum amount of S2Cl2 produced when 4.06 g of S8 is reacted with 6.24 g of Cl2? If the actual yield of S2Cl2 is 6.55 g, what is the percent yield for this reaction?
Given: 4.06 g S8, 6.24 g Cl2
Actual yield of S2Cl2: 6.55 g
Calculating the theoretical yield of S2Cl2 is a limiting reagent problem. Calculate how much S2Cl2 that each reactant could make provided there is enough of the other reactant around:
Since S8 produces the lesser amount of S2Cl2, it is the limiting reactant and gives the amount of S2Cl2 produced. Converting from moles to grams:
13. (10 points) Calculate the heat required to raise the temperature of 2.25 kg of water from 5.0°C to 92.5°C. The specific heat of water is equal to 4.184 J/g·°C.
The equation to use is: q = (specific heat)×(mass in grams)×DT
Calculate DT:
DT = 92.5°C – 5.0°C = 87.5°C
Calculate the mass in grams of water:
Mass in g = (2.25 kg)(1000 g/kg) = 2250 g
The heat required is equal to:
q = (4.184 J/g·°C)(2250 g)(87.5°C) = 824,000 J (3 sig figs)
14. (10 points) Write the net ionic equations for each of the following reactions:
(a) 2 Al(s) + 3 NiCl2(aq) ¾¾® 2 AlCl3(aq) + 3 Ni(s)
The total ionic equation is:
2 Al(s) + 3 Ni2+(aq) + 2 Cl¯(aq) ¾¾® 2 Al3+(aq) + 3 Cl¯(aq) + 3 Ni(s)
The spectator ion is Cl¯. Removing the spectator ion, the net ionic equation is:
2 Al(s) + 3 Ni2+(aq) ¾¾® 2 Al3+(aq) + 3 Ni(s)
(b) Fe(s) + 2 HNO3(aq) ¾¾® Fe(NO3)2(aq) + H2(g)
The total ionic equation is:
Fe(s) + 2 H+(aq) + 2 NO3¯(aq) ¾¾® Fe2+(aq) + 2 NO3¯(aq) + H2(g)
The spectator ion is NO3¯. Removing the spectator ion, the net ionic equation is:
Fe(s) + 2 H+(aq) ¾¾® Fe2+(aq) + H2(g)
(c) Na2CO3(aq) + MgSO4(aq) ¾¾® MgCO3(s) + Na2SO4(aq)
The total ionic equation is:
2 Na+(aq) + CO32-(aq) + Mg2+(aq) + SO42-(aq) ¾¾® MgCO3(s) + 2 Na+(aq) + SO42-(aq)
The spectator ions are Na+ and SO42-. Removing the spectator ions, the net ionic equation is:
CO32-(aq) + Mg2+(aq) ¾¾® MgCO3(s)
(5 points extra credit) Only one of the above 3 reactions in question 14 is not an oxidation-reduction reaction. Please identify by the letter of the reaction, which one of the above 3 reactions is not an oxidation-reduction reaction. Put only one choice down.
The 3rd reaction in (c) is a precipitation reaction and not an oxidation-redution reaction.
15. (15 points) The concentration of iron(II) ion in a solution may be determined by reaction with Cr2O22- ion according to the reaction:
Cr2O72-(aq) + 6 Fe2+(aq) + 14 H+(aq) ¾¾® 2 Cr3+(aq) + 6 Fe3+(aq) + 7 H2O(l)
___ If it required 26.0 mL of 0.0250 M Cr2O72- to fully react with all of the Fe2+ ion in 25.0 mL of a solution, what is the molarity of Fe2+ in that solution?
Known: 26.0 mL of 0.0250 M Cr2O72-
Unknown: 25.0 mL of ?? M Fe2+
Calculate the number of moles of Cr2O72-:
Convert to moles of Fe2+:
Calculate molarity of Fe2+ solution:
