CHAPTER 10.3 REVIEW KEY
10.3 Worksheet
1.
Percent composition
2.
100
3.
Molar mass
4.
Empirical
5.
Whole-number
6.
Molecular
7.
ST (You can use experimental data too)
8.
AT
9.
AT
10.
NT
11.
C
12.
B
13.
A
(It is CH2O)
14. The first problem will be worked out in detail
a. Step 1: Calculate the molar mass of the elements in the compound and the molar mass of the
compound.
Cr2
= 52.0 x 2
= 104 g
O3
= 16.0 x 3
= + 48.0 g
Cr2O3
c.
% of Hg
=
86.3%
% of S
=
13.7%
= 152 g (molar mass)
Step 2: Divide the molar mass of each element in the compound by the molar mass of the
compound, then multiply by 100 to convert to percent.
% of Cr in Cr2O3 =
104 g Cr
X 100 =
152 g Cr2O3
68.4%
% Comp of O =
48 g O
X 100 =
152 g Cr2O3
31.6%
CHAPTER 10.3 REVIEW KEY
b. % of Mn = 38.7%
d.
% of Ca = 24.5%
% of P
= 21.8%
% of N = 17.1%
% of O
= 39.4%
% of O = 58.5%
15. Step 1: Assume 100 g of the compound, so 29.1% Na will become 29.1 g Na, 40.5% S will become
40.5 g S, and 30.4% O will become 30.4 g O.
Step 2: Calculate the number of moles of each element in 100 g of the compound.
29.1 g Na
1.00 mol Na
=
X
1
23.0 g Na
1.27 mol Na
40.5 g S
1
X
1.00 mol S
32.1 g S
=
1.27 mol S
30.4 g O
1
X
1.00 mol O
16.0 g O
=
1.90 mol O
Smallest mole number
Step 3: Divide the moles of each element by the smallest number of moles to determine the mole
ratio of each element in the compound.
Na
1.27 mol
1.27 mol
:
1
:
:
S
1.27 mol
1.27 mol
:
1
:
:
O
1.90 mol
1.27 mol
1.5
Na1S1O1.5
Step 4: You can’t have the fraction 1.5, so multiply each number in the ratio by 2.
2 x Na1S1O1.5 = Na2S2O3
16. Step 1: Calculate the molar mass of iron and the molar mass of Fe2O3.
Fe2
O3
= 55.8 x 2 = 112 g
= 16.0 x 3 = + 48 g
Fe2O3
= 160 g (molar mass)
Step 2: Calculate the percent composition of iron in Fe2O3.
% Comp of Fe =
112 g Fe
160 g Fe2O3
X 100 =
70 %
Step 3: Determine number of kg of iron that makes up 639 kg of Fe2O3.
639 kg Fe2O3 x 70% = 447 kg Fe