Answer Key
Chapter 9: Standard Review Worksheet
1. Answers will vary. An example is included below:
2H2O2(aq)
2H2O(l) + O2(g)
This describes the decomposition reaction of hydrogen peroxide.
Microscopic: Two molecules of hydrogen peroxide (in aqueous solution) decompose to
produce two molecules of liquid water and one molecule of oxygen gas.
Macroscopic: Two moles of hydrogen peroxide (present in aqueous solution) decompose to
produce two moles of liquid water and one mole of oxygen gas.
2. The mole ratios for a reaction are based on the coefficients of the balanced chemical
equation for the reaction. These coefficients show in what proportions molecules (or moles
of molecules) combine. From the balanced equation
C2H5OH(l) + 3O2(g)
2CO2(g) + 3H2O(g)
(and assuming a given amount of C2H5OH) various mole ratios can be constructed.
2 mol CO2
To calculate mol CO2 produced:
1 mol CHOH
25
To calculate mol H2O produced:
3 mol HO
2
1 mol CHOH
25
To calculate mol O2 required:
3 mol O
2
1 mol CHOH
25
We could calculate the number of moles of the other substances if 0.65 mol C2H5OH were
to be combusted as follows:
Mol CO2 produced = (0.65 mol C2H5OH) _ 2 mol CO2 = 1.3 mol CO2
1 mol CHOH
25
Mol H2O produced = (0.65 mol C2H5OH) _
Mol O2 required = (0.65 mol C2H5OH) _
3 mol HO
2
1 mol CHOH
25
3 mol O
2
1 mol CHOH
25
= 1.95 = 2.0 mol H2O
= 1.95 = 2.0 mol O2
3. It is the limiting reactant that controls how much product is formed, regardless of how
much of the other reactants are present. The limiting reactant limits the amount of product
that can form in the experiment because once the limiting reactant has reacted completely,
the reaction must stop.
4.
a. 2AgNO3(aq) + CaSO4(aq) Ag2SO4(s) + Ca(NO3)2(aq)
Molar masses: AgNO3, 169.9 g; Ag2SO4, 311.9 g; Ca(NO3)2, 164.1 g
25.0 g AgNO3 _ 1 mol = 0.147 mol AgNO3
169.9 g
0.147 mol AgNO3 _
1 mol AgSO
24
2 mol AgNO
3
0.0735 mol Ag2SO4 _
0.147 mol AgNO3 _
311.9 g
=
1 mol
= 0.0735 mol Ag2SO4
22.9 g Ag2SO4
1 mol Ca(NO)
32
2 mol AgNO
3
0.0735 mol Ca(NO3)2 _
164.1 g
=
1 mol
= 0.0735 mol Ca(NO3)2
12.1 g Ca(NO3)2
b. 2Al(s) + 6HNO3(aq) 2Al(NO3)3(aq) + 3H2(g)
Molar masses: Al, 26.98 g; Al(NO3)3, 213.0 g; H2, 2.016 g
25.0 g Al _ 1 mol = 0.927 mol Al
26.98 g
0.927 mol Al _
2 mol Al(NO)
33
2 mol Al
= 0.927 mol Al(NO3)3
213.0 g
= 197 g Al(NO3)3
1 mol
0.927 mol Al _ 3 mol H2 = 1.39 mol H2
2 mol A
2.016 g
1.39 mol H2 _
= 2.80 g H2
1 mol
0.927 mol Al(NO3)3 _
c. H3PO4(aq) + 3NaOH(aq) Na3PO4(aq) + 3H2O(l)
Molar masses: H3PO4, 97.99 g; Na3PO4, 163.9 g; H2O, 18.02 g
25.0 g H3PO4 _ 1 mol = 0.255 mol H3PO4
97.99
0.255 mol H3PO4 _
1 mol NaPO
34
1 mol HPO
34
= 0.255 mol Na3PO4
163.9 g
= 41.8 g Na3PO4
1 mol
3 mol HO
2
= 0.765 mol H2O
1 mol HPO
34
0.255 mol Na3PO4 _
0.255 mol H3PO4 _
0.765 mol H2O _
18.02 g
=
1 mol
13.8 g H2O
d. CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)
Molar masses: CaO, 56.08 g; CaCl2, 111.0 g; H2O, 18.02 g
25.0 g CaO _ 1 mol = 0.446 mol CaO
56.08
0.446 mol CaO _
1 mol CaCl
2
1 mol CaO
= 0.446 mol CaCl2
111.0 g
= 49.5 g CaCl2
1 mol
1 mol HO
2
= 0.446 mol H2O
1 mol CaO
0.446 mol CaCl2 _
0.446 mol CaO _
0.446 mol H2O _
18.02 g
=
1 mol
8.04 g H2O
5.
a. 2AgNO3(aq) + CaSO4(aq) Ag2SO4(s) + Ca(NO3)2(aq)
Molar masses: AgNO3, 169.9 g; CaSO4, 136.2 g; Ag2SO4, 311.9 g;
Ca(NO3)2, 164.1 g
12.5 g AgNO3 _ 1 mol = 0.0736 mol AgNO3
169.9 g
10.0 g CaSO4 _
1 mol
=
136.2 g
0.0734 mol CaSO4
AgNO3 is the limiting reactant.
24
0.0736 mol AgNO3 _ 1 mol AgSO
_
2 mol AgNO
3
0.0736 mol AgNO3 _
1 mol Ca(NO)
32
2 mol AgNO
3
311.9 g AgSO
24
=
1 mol AgSO
24
_
164.1 g
=
1 mol
11.5 g Ag2SO4
6.04 g Ca(NO3)2
b. 2Al(s) + 6HNO3(aq) Al(NO3)3(aq) + 3H2(g)
Molar masses: Al, 26.98 g; HNO3, 63.02 g, Al(NO3)3, 213.0 g; H2, 2.016 g
12.5 g Al _ 1 mol = 0.463 mol Al
26.98 g
10.0 g HNO3 _
1 mol
63.02
= 0.159 mol HNO3
HNO3 is the limiting reactant.
33
0.159 mol HNO3 _ 2 mol Al(NO)
_
6 mol HNO3
0.159 mol HNO3 _
3 mol H2
6 mol HNO3
_
213.0 g
=
1 mol
2.016 g
=
1 mol
11.3 g Al(NO3)3
0.160 g H2
c. H3PO4(aq) + 3NaOH(aq) Na3PO4(aq) + 3H2O(l)
Molar masses: H3PO4, 97.99 g; NaOH, 40.00 g, Na3PO4, 163.9 g; H2O, 18.02 g
12.5 g H3PO4 _ 1 mol = 0.128 mol H3PO4
97.99
10.0 g NaOH _
1 mol
=
40.00 g
0.250 mol NaOH
NaOH is the limiting reactant.
34
0.250 mol NaOH _ 1 mol NaPO
_
0.250 mol NaOH _
163.9 g
= 13.7 g Na3PO4
1 mol
3 mol NaOH
3 mol HO
2
_ 18.02 g= 4.51 g H2O
3 mol NaOH
1 mol
d. CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)
Molar masses: CaO, 56.08 g; HCl, 36.46 g, CaCl2, 111.0 g; H2O, 18.02 g
12.5 g CaO _ 1 mol = 0.222 mol CaO
56.08
10.0 g HCl _
1 mol
36.458
= 0.274 mol HCl
Since twice as many moles of HCl (compared with CaO) are required, HCl is the
limiting reactant.
0.274 mol HCl _ 1 mol CaCl2 _ 111.0 g= 15.2 g CaCl2
0.274 mol HCl _
1 mol
2 mol HCl
1 mol HO
18.02
g
2
_
=
1 mol
2 mol HCl
2.47 g H2O
6. There is no pattern. For example, in part a, AgNO3 is present with the highest mass and
greatest number of moles, and it is the limiting reactant. In part b, HNO3 limits the reaction
and is present in the least amount (of mass and moles). Students need to understand that
they must figure out the limiting reactant, not just memorize an incorrect shortcut such as
“the limiting reactant is present in least amount.”
7. We determine the theoretical yield by stoichiometric calculations. The actual yield is
determined by experiment. The percent yield is calculated by taking the actual yield,
dividing by the theoretical yield, and multiplying this number by 100%.
8. (2.8 g)/(4.0 g) _ 100% = 70.% yield.