Un.it2
C.3 SUPPLEMENT:
MOLAR MASS COMPUTATION AND CONVERSIONS
Molar Mass Computations
The molar mass of a substance is the mass Of one mole (6.02 X 1023 units)
of any
substance. For the purpose of this course, we will use the nearest whole-numbe
r
vahw of the atomic masses found on the Periodic Table.
To find the molar mass of a substance, multiply the number of atoms of each
element by the atomic mass of the element. Then add the masses of the
various
elements.
Example 1: What is the molar mass of iron(Ill) oxide, Fe
?
3
0
2
2Fe=2X56=112
30 =3x16= 48
3
Pe0
160 g/mol
Example 2: What is the molar mass of magnesium hydroxide, ?
2)
Mg(OH
1Mg = 1’X24 = 24
20 =2X16=32
2H =2X1 = 2
Mg(OH)
2
= 58 glmol
Determine the molar mass of each substance.
1. Nitrogen gas: N
2
2. SOdium chloride (table salt): NaC1
3. Sucrose (table sugar): 0
C
2
H
1
2
1
4. Chalcopyrite: CuFeS
2
S. Malachite: 3
CO (OH)
2
Cu
2
Mole-Mass Conversions
Once the molar mass of a substance is known, it is fairly easy to calculate moles
from grams or grams from moles.
?
3
0
2
Example 3: How many moles are present in 352 g of iron(III) oxide, Fe
This can be expressed in two ways. It is either
a.
3 contains 1 mole (160 gIl mol) or
0
2
160 g of Fe
b.
3 contains 160 g (1 mol/160 g)
0
2
1 mole of Fe
Either fraction may be used as a conversion factor so that the units of a calcula
tion will cancel.
Step 1:
Calculate the molar mass of iron(I1l) oxide. From Example 1, it
is 1 mol = 160 g.
Step 2:
Arrange the molar mass so the original units cancel and the
desired units are on top.
352g x
160g
=
2,2molofFe.O
2 3
Example 4: How many grams are present in 1.43 moles of magnesium hydroxide,
?
2
Mg(OH)
Step 1:
Calculate the molar mass of magnesium hydroxide. From
Example 2, it is 1 mol = 58 g.
Step 2:
Arrange the molar mass so the original units cancel and the
desired units are on top.
1.43mc1
X
=
2
83gofMg(OH)
Calculate the following. (Hint: The molar masses are from Questions 1—6.)
6.
1.0 mol of Azurite, 2
(OH)
Cu
(
)
3
,
C0 is how many grams?
2 is how many grams?
7. 6.3 mol of N
8.
84.6 g of NaCI is how many moles?
C
2
H
1
2 is how many moles?
1
9. 564 g of 0
10. 3.95 mol of CuFeS
2 is how many grams?
ii,
0.985 g of 3
CO (OH)
2
Cu
2 is how many iiiole?
12.
36.5 mol of 2
C
(
(
)
3
QH
C0
u
) is how many grams?
Unit2
CA SUPPLEMENT: PERCENT COMPOSITI
ON
OF MATERIALS AND CONSERVATION
OF MASS
Example: What percent of iron(III) hydroxide,
3) is oxygen
Fe(OH
,
Step 1: Find the molar
mass of the compound.
iFe
=
56g
30
3X16 = 48g
V
.
LVV.
107 g/mol
Step 2:
V
Find the percentage by dividing the part
by the whole and
multiplying by 100.
V
48 g oxygen
X 100
total
1.
2.
=
45%
What percent of magnesium bromide,
, is magnesium?
2
MgBr
What percent of glucose, 0
12 is carbon?
H
C
,
6
3. What percent is zinc of Zn
3 (P0
?
2
)
4
V
3
6. What percent of AgNO
is silver?
?
)
3
3 or Al (NO
0
2
s. Which has the higher percent of aluminum, A1
Balance these equations: Mathematically show that the law of conservation of
matter is upheld.
Example: Zn + HCI
lZn
1H
1CI
•
Zn+2HC1
—
2
ZnCI. + H
2
I
2
2
+H
2
ZnCI
Step 1:
Balance the equation.
Step 2:
Find molar masses, multiply by coefficient or subscripts and
mathematically prove that reactants = products.
65+Z(1+35)=65+2(35)+2(1)
65+72=135+2
137g = 137g
C1
cl—A1
H
2
tAl+H
+
3
.
I
NW
+Nil
2
F
—
5
+
2. 3
3. Fe +02
4.
3
0
2
Fe
3
3 — NH
CI + Fe(OH)
4
H + FeCI
NH
O
4