GENERAL CHEMISTRY I
CHEM 1311.001
EXAM 3
Monday March 29, 1999

Name________________________________________

SSN_________________________________________

1. What percent of the mass of aluminum oxide, Al2O3 is aluminum? Atomic weights of the elements encountered in this problem are as follows:

Al = 26.981539            O = 15.9994
a) 26.46% b) 40.00% c) 52.93% d) 59.30% e) 62.78%
2. A compound known to contain only the elements zinc and oxygen was found to be 80.34% zinc by mass. What is the empirical formula of this compound? Atomic weights of the elements encountered in this problem are as follows:

Zn = 65.39            O = 15.9994
a) ZnO b) Zn2O c) ZnO2 d) Zn3O e) ZnO3
3. A chemist was working with a compound known to contain only the elements carbon and hydrogen. A 5.000 g sample of this compound was analyzed and found to contain 3.994 g of carbon. What is the empirical formula of this compound? Atomic weights of the elements encountered in this problem are as follows:

C = 12.011            H = 1.00794
a) CH b) C2H c) CH2 d) C3H e) CH3
4. Methane (CH4) is a principal ingredient in the "natural gas" used for heating and cooking in many American homes. This gas can be produced synthetically from carbon monoxide and hydrogen at high temperatures. The reaction is

     CO(g)      +      3H2(g)      ---------->      CH4(g)      +      H2O(g)

If a chemist had 9.912 g of CO and a large excess of H2 available, what is the maximum possible mass of CH4 that could be produced? Atomic weights of the elements encountered in this problem are as follows:

C = 12.011            O = 15.9994            H = 1.00794
a) 0.354 g b) 1.892 g c) 3.304 g d) 5.677 g e) 9.912 g
5. The reaction in problem 4 produces steam, H2O(g) as a by-product. Assuming all of the CO reacted in problem 4, what mass of steam was produced? Atomic weights of the elements encountered in this problem are as follows:

C = 12.011            O = 15.9994            H = 1.00794
a) 1.594 g b) 2.125 g c) 3.188 g d) 6.375 g e) 9.912 g
6. Assuming all the CO reacted in problem 4, what mass of H2 was used? Note: You can calculate this either by stoichiometry, or by conservation of mass. To use conservation of mass, note that the mass of CO was given, and you have calculated the mass of CH4 (in problem 4) and H2O (in problem 5). If time permits, you might want to solve it both ways as a check on your work. Atomic weights of the elements encountered in this problem (for the stoichiometry calculation) are as follows:

C = 12.011           O = 15.9994           H = 1.00794
a) 0.357 g b) 0.713 g c) 1.062 g d) 2.140 g e) 9.912 g
7. Which reactant determines the amount of product(s) that can be formed in a chemical reaction?
a) the limiting reactant b) the excess reactant
8. True or False: The reactant with the smallest mass is always the limiting reactant.
a) True b) False
9. The reactant from which you calculate the larger amount of product is the
a) limiting reactant b) excess reactant
10. The air around us is 21% oxygen, O2, 78% nitrogen, N2 and 1% a mixture of various other gases. The N2 and O2 do not react with each other at room temperature, but at high temperature they react as follows:

     N2(g)      +      O2(g)      ---------->      2NO(g)

In an experiment, a chemist heated a gaseous mixture containing 5.113 g of N2 and 8.976 g of O2 in a steel reaction vessel. What is the maximum possible amount (theoretical yield) of NO that can be formed in this reaction? Atomic weights of the elements encountered in this problem are as follows:

N = 14.00674            O = 15.9994
a) 5.477 g b) 8.417 g c) 10.953 g d) 14.089 g e) 16.834 g
11. Which reactant did you find was the limiting reactant in problem 10?
a) N2 b) O2 c) neither -- it was a stoichiometric mixture
12. Assuming the theoretical yield of NO was obtained in problem 10, what mass of N2 was left over (not used in the reaction)?
a) 0.000 g b) 1.184 g c) 2.556 g d) 3.106 g e) 3.863 g
13. Assuming the theoretical yield of NO was obtained in problem 10, what mass of O2 was left over (not used in the reaction)?
a) 0.000 g b) 2.920 g c) 3.136 g d) 5.840 g e) 6.056 g
14. For the reaction in problem 10, if the actual yield of NO was 2.087 g, what was the percent yield of the reaction?
a) 12.40% b) 14.81% c) 19.05% d) 24.80% e) 38.10%

***      END      OF      TEST      ***
 

BRIEF ANSWERS

1 c            2 a            3 e            4 d            5 d

6 d            7 a            8 b            9 b            10 c

11 a            12 a            13 c            14 c

DETAILED ANSWERS

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