Stoichiometry Worksheet 2: Percent Yield

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Date
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Stoichiometry Worksheet 2: Percent Yield
For each of the problems below:
a. Write the balanced chemical equation
b. Identify the given (with units) and what you want to find (with units)
c. Show set up with units. Check sig figs, give final answer with units and label.
1. Using the Hoffman apparatus for electrolysis, a chemist decomposes 36 g of water
into its gaseous elements. How many grams of hydrogen gas should she get
(theoretical yield)?
Equation:
2 H2O(l)
2 H2(g) +
1 O2 (g)
Before
2 mol
0 mol
Change
-2 mol
+2 mol
After
0 mol
2 mol
36 g H 2O x
1 mol H 2O
18.0 g H 2O
0 mol
2.0 mol H2O
2 mol H2
2 mol H2O
2 mol H2
2.0 g H 2
1 mol H2
4.0 g H 2
2. Recall that liquid sodium reacts with chlorine gas to produce sodium chloride. You
want to produce 581 g of sodium chloride. How many grams of sodium are needed?
Equation:
2 Na(l) + 1 Cl2(g)
2 NaCl(s)
Before
9.94 mol
Change
-9.94 mol
+9.94 mol
After
0 mol
9.94 mol
581 g NaCl x
xs mol
1 mol NaCl
58.5g NaCl
0 mol
9.93 mol NaCl
2 mol Na
2 mol NaCl
9.93 mol Na
23.0 g Na
1 mol Na
228 g Na
3. You eat 180.0 g of glucose (90 M&Ms). If glucose, C6H12O6, reacts with oxygen gas
to produce carbon dioxide and water, how many grams of oxygen will you have to
breathe in to burn the glucose?
Equation:
1 C6H12O6(s) +
6 O2(g)
6 CO2(g) + 6 H2O(g)
Before
1.0 mol
xs mol
Change
-1.0 mol
-1.0 mol
After
0 mol
xs mol
180.0 g C6H12O 6 x
6.0 mol O2 x
1 mol C 6H12O 6
180 g C 6H12O 6
32.0 g O 2
1 mol O 2
0 mol
1.00 mol C 6H12O 6
0 mol
6 mol O2
1 mol C6H12O 6
6.00 mol O2
192 .0 g O2
©Modeling Instruction – AMTA 2014
1
U8 ws2 v2.0
4. Suppose 4.61 g of zinc was allowed to react with hydrochloric acid to produce zinc
chloride and hydrogen gas. How much zinc chloride should you get?
Suppose that you actually recovered 8.56 g of zinc chloride. What is your percent
yield? From here on, moles are understood in the BCA table.
Equation:
1 Zn(s)
Before
0.0705
Change
-0.0705
After
4.61 g Zn
+
2 HCl(aq)
xs
+
0
1 H2 (g)
0
+0.0705
0
1 mol Zn
65.4 g Zn
1 ZnCl2(aq)
0.0705
1 mol ZnCl2
1 mol Zn
0.0705 mol Zn
8.56 g ZnCl 2 actual
9.61 g ZnCl 2 theoretical
0.0705 mol ZnCl2
136.3 g ZnCl2
1 mol ZnCl2
9.61 g ZnCl2
100% 89.1% yield
5. Determine the mass of carbon dioxide that should be produced in the reaction
between 3.74 g of carbon and excess O2. What is the % yield if 11.34 g of CO2 is
recovered?
Equation:
1 C(s) +
1 O2(g)
1 CO2(s)
Before
0.311
xs
0
Change
-0.311
-0.311
+0.311
After
3.74 g C
0
1 mol C
12.0 g
11.34 g CO2
13.7 g CO2
xs
0.311 mol C
0.311
1 mol CO2
1 mol C
0.311 mol CO2
44.0 g CO2
1 mol CO2
13.7 g CO2
100 % 82.8 % yield
6. In the reaction between excess K(s) and 4.28 g of O2(g), potassium oxide is formed .
What mass would you expect to form (theoretical yield)? If 17.36 g of K2O is
actually produced, what is the percent yield?
Equation:
4 K(s) +
1 O2(g)
2 K2O(s)
Before
xs
0.134
0
Change
-0.536
-0.134
+0.268
After
xs
0
0.268
4.28 g O2
1 mol O2
32.0 g O2
17.36 g K 2 O
100 %
25.2 g K 2 O
0.134 mol O2
2 mol K 2 O
1 mol O2
0.268 mol K 2 O
94.2 g K 2 O
1 mol K 2 O
25.2 g K 2 O
68.9 % yield
©Modeling Instruction – AMTA 2014
2
U8 ws2 v2.0
7. Determine the mass of carbon dioxide one could expect to form (and the percent
yield) for the reaction between excess CH4 and 11.6 g of O2 if 5.38 g of carbon
dioxide gas is produced along with some water vapor.
Equation:
1 CH4(g) + 2 O2(g)
1 CO2(g) + 2 H2O(g)
Before
xs
Change
-0.363
After
11.6 g O2
0.363
xs
1 mol O2
32.0 g O2
5.38 g CO2
7.98 g CO2
100 %
0
0.363 mol O2
0
0
+0.181
0.181
1 mol CO2
2 mol O2
0.181 mol CO2
44.0 g CO2
1 mol CO2
7.98 g CO2
67.4 % yield
8. Determine the mass of water vapor you would expect to form (and the percent yield)
in the reaction between 15.8 g of NH3 and excess oxygen to produce water and
nitric oxide (NO). The mass of water actually formed is 21.8 g.
Equation:
4 NH3(g) + 5 O2(g)
6 H2O(g) + 4 NO(g)
Before
0.928
Change
-0.928
+1.39
After
0
1.39
15.8 g NH3
1 mol NH3
17.0 g NH3
21.8 g H 2 O
100 %
25.1 g H 2 O
xs
0.928 mol NH3
0
0
6 mol H2O
1.39 mol H 2O
4 mol NH3
25.1 g H2O
86.9 % yield
Check your answers (1) 4.0 g H2
(5) 13.7 g CO2, 82.8%
18.0 g H 2O
1 mol H 2O
(2) 229 g Na
(6) 25.2 g K2O, 68.9%
©Modeling Instruction – AMTA 2014
3
(3) 192 g O2
(4) 9.61 g Zn, 89.1%
(7) 7.98 g CO2, 67.4% (8) 25.1 g H2O, 86.9%
U8 ws2 v2.0