Dimensional Analysis, Sig Figs, and the Mole

1
Unit VI

• Dimensional Analysis, Sig Figs, and the Mole

2
Obj. 1Formula vs. Molecular Mass

• Formula mass total mass of ionic compounds

- starts with a metal

• Molecular mass total mass of molecular
  compounds

- starts with a non-metal

• To calculate

- find mass of each element on P.T. (whole s)
- multiply mass by subscript and/or coefficient
- add all masses together for total mass
3
Obj. 1 cont

• Practice

Al2(SO4)3
S3F6
27 x 2 54
16 x 12 192
19 x 6 114
32 x 3 96
32 x 3 96
210 amu
96 114
54 96 192
342 amu
molecular mass
formula mass
4
Obj. 1 cont
two compounds chemically bonded together
NOT multiply!!!

• more practice

CuSO4 ? 5H2O
3CaC2O4
64
32
64
80
10
16 x 12 192
40 x 3 120
OR
16 x 4 64
40 x 1 40
64 32 64 10 80
250 amu
12 x 6 72
12 x 2 24
120 72 192
384 amu
40 24 64
128
x 3
384 amu
5
Obj. 2The Mole

• a pair

2 (shoes)

• a dozen

12 (eggs)

• a score

20 (years)

• A mole describes a chemical quantityits just a
  number!!!

1 mole 6.02 x 1023 units Avagadros

• you can have a mole of anythingshoes, cars,
  people etc

6
Obj. 2 cont

• recall that one p / n0 1.67 x 10-24 grams

• thereforeone mole of nucleons 1 gram

1.67E -24 x 6.02E23 1

• amu g/mole

molar mass
atomic mass

• one mole

6.02 x 1023 atoms, molecules or ions in a
compound
molar mass of compound / element
22.4 liters of any gas at STP
(standard temp. and pressure)
760 mmHg
0C
101.3 kPa
273K
1 atm
7
Obj. 3-7Mole Conversions

• use mole map to solve mole conversions

going away from moles x
going toward moles
? by molar mass
X by 22.4
Mole
Liters
Grams
? by 22.4
X by molar mass
? by 6.02 x 1023
X by 6.02 x 1023
of atoms/ molecules
8
Obj. 3-7 cont

• Practice, practice, practice

1.2 x 1024
2.0 moles He ____________ atoms He
2.0 moles x 6.02E23
2
16
18
36
2.0 moles H2O ____________ grams H2O
2.0 moles x 18
3.00
67.2 liters O2 ____________ moles O2
67.2 liters 22.4
2
16
18
1.5
27 grams H2O ____________ moles H2O
27 grams 18
9
Obj. 3-7 cont

• more practice

6.00
3.61 x 1024 molecules NaCl ____________ moles
NaCl
3.61 x 1024 molecules 6.02 x 1023
110
5.0 moles CO2 ____________ liters CO2
5.0 moles x 22.4
32
44
12
264
3.61 x 1024 molecules CO2 ____________
grams CO2
44 grams
3.61 x 1024 molecules 6.02 x 1023 6 moles x
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Obj. 8-9 Composition

• composition is the mass that one particular
  element

contributes to an entire compound.

• to calculate

( part / whole)
find total mass of one molecule
mass of element (part) by total mass
(whole)
x by 100

• practice

4
14
80 amu
14
48

• what is the of oxygen in NH4NO3?

x 100
0.6
60
oxygen 48 / 80
x 100
0.35
35
28 / 80
nitrogen
11
Obj. 8-9 cont

• hydrated compounds(chemically bonded to water)

calculate total mass of molecule (including
water)
mass of water by total mass
10
80
250 amu
32
64
64

• what is the of water in CuSO4 ? 5H2O

water 90 / 250
x 100
0.36
36
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Obj. 10-11Empirical Formulas

• empirical formulas are the lowest whole ratio
  of a

compound.

• i.e. the empirical formula for glucose (C6H12O6)
  would be

CH2O

• to calculate from composition

1. given by atomic mass (to find moles)
2. each mole in step 1 by lowest mole value
(to find ratio)
3. values from step 2 subscripts
subscripts MUST be a whole
(multiply if they are not!)
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Obj. 10-11 cont

• practice

1. What is the empirical formula of a compound
that
contains 36.5 sodium, 25.4 sulfur and 38.1
oxygen?
Na 36.5
1.59 moles
2
23
0.794
S 25.4
0.794 moles

Na SO
1
subscripts
32
3
2
0.794
O 38.1
2.38 moles
3
16
0.794
14
Obj. 10-11 cont

• more practice

2. What is the empirical formula of a compound
that
contains 72.4 iron (Fe) and 27.6 oxygen?
Fe 72.4
1.293 moles

1
x 3
3
56
1.293
O 27.6
1.725 moles
1.33
x 3
4
16
1.293
MUST be a whole !!!
Fe O
4
3
15
Obj. 10-11 cont

• calculate hydrated compounds (bonded to water)
  same way

use ? to separate water from crystal
since water is a compound, you are finding the
coefficient
NOT the subscript!

• practice

What is the formula of a compound that contains
18.28 Ca,
32.36 Cl and 49.36 H2O?
Ca 18.28
0.457 moles

1
40
0.457
Cl 32.36
CaCl ? H2O
6
0.925 moles
2
2
35
0.457
H2O 49.36
2.742 moles
6
18
0.457