1. Dimensional Analysis, Sig Figs, and the Mole
Unit VI
Dimensional Analysis, Sig Figs, and the Mole

2. Obj. 1…Formula 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… Al2(SO4)3 S3F6 96 + 114 = 210 amu 54 + 96 + 192 = 342 amu
Practice…
Al2(SO4)3
S3F6
27 x 2 = 54
16 x 12 = 192
32 x 3 = 96
19 x 6 = 114
32 x 3 = 96 =
210 amu =
342 amu
molecular mass
formula mass

4. two compounds chemically bonded together… NOT multiply!!!
Obj. 1 cont…
two compounds chemically bonded together… NOT multiply!!!
more practice…
CuSO4  5H2O
3CaC2O4 64 32 64 10 80
40 x 3 = 120
16 x 12 = 192 OR
16 x 4 = 64
40 x 1 = 40 =
250 amu
12 x 6 = 72
12 x 2 = 24 =
384 amu =
128
x 3 =
384 amu

5. 1 mole = 6.02 x 1023 units = Avagadro’s #
Obj. 2…The Mole
a pair =
2 (shoes)
a dozen =
12 (eggs)
a score =
20 (years)
A mole describes a chemical quantity…it’s just a number!!!
1 mole = 6.02 x 1023 units = Avagadro’s #
you can have a ‘mole’ of anything…shoes, cars, people etc…

6. Obj. 2 cont… recall that one p+ / n0 = 1.67 x 10-24 grams
therefore…one mole of nucleons = 1 gram
** 1.67E -24 x 6.02E23 = 1
amu = g/mole
atomic mass
molar 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
0°C
101.3 kPa
273K
1 atm

7. Obj. 3-7…Mole Conversions
use ‘mole map’ to solve mole conversions…
** going away from moles = x
** going toward moles = ÷
X by 22.4
 by molar mass
Liters
Mole
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
1.2 x 1024
2.0 moles x 6.02E23 = 2 16
= 18
2.0 moles H2O = ____________ grams H2O 36
2.0 moles x 18 =
67.2 liters O2 = ____________ moles O2
3.00
67.2 liters ÷ 22.4 = 2 16
= 18
27 grams H2O = ____________ moles H2O
1.5
27 grams ÷ 18 =

9. Obj. 3-7 cont… more practice… 6.00
3.61 x 1024 molecules NaCl = ____________ moles NaCl
6.00
3.61 x 1024 molecules ÷ 6.02 x 1023 =
5.0 moles CO2 = ____________ liters CO2
110
5.0 moles x 22.4 = 12 32
= 44
** 3.61 x 1024 molecules CO2 = ____________ grams CO2 **
264
3.61 x 1024 molecules ÷ 6.02 x 1023 = 6 moles x
44 grams =

10. 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… 14 4 14
48 =
80 amu
what is the % of oxygen in NH4NO3?
% oxygen = 48 / 80 =
0.6
x 100 =
60%
% nitrogen =
28 / 80 =
0.35
x 100 =
35%

11. Obj. 8-9 cont… hydrated compounds…(chemically bonded to water)
~ calculate total mass of molecule (including water)
~ ÷ mass of water by total mass 64 32 10
80 =
250 amu 64
what is the % of water in CuSO4  5H2O
% water = 90 / 250 =
0.36
x 100 =
36%

12. Obj. 10-11…Empirical 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!)

13. Obj. 10-11 cont… Na SO *** 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%
= moles
***
= 1
Na SO 32
subscripts
0.794 2 3
O = 38.1%
= 2.38 moles
= 3 16
0.794

14. Obj. 10-11 cont… x 3 = 3 x 3 = 4 Fe O *** more practice…
2. What is the empirical formula of a compound that
contains 72.4% iron (Fe) and 27.6% oxygen?
Fe = 72.4%
= moles
***
= 1
x 3 = 3 56
1.293
O = 27.6%
= moles
= 1.33
x 3 = 4 16
1.293
MUST be a whole #!!!
Fe O 3 4

15. Obj 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%
= moles
***
= 1 40
0.457
Cl = 32.36%
= moles
CaCl  H2O 6
= 2 2 35
0.457
H2O = 49.36%
= moles
= 6 18
0.457