Draw the Lewis Structure and give
the molecular shape and molecular
polarity of the following molecules:
H2S
2. CHCl3
3. SO3
4. O2
1.
Give the name or formula of the
following provided the opposite.
1.
2.
3.
4.
5.
H3PO4
CF4
NaNO3
C2H4
SO2
6.
7.
8.
9.
10.
Methane
Nitrogen trihydride
Ammonium hydroxide
Ethanol
Hydrochloric acid
A
counting number (like a dozen)
 Avogadro’s
1
number (NA)
mol = 6.02  1023 particles
A
large amount!!!!

1 mole of hockey pucks would
equal the mass of the moon!

1 mole of basketballs would fill
a bag the size of the earth!

1 mole of pennies would cover
the Earth 1/4 mile deep!
 Mass
of 1 mole of an element or
compound.
 Atomic
mass tells the...
› atomic mass units per atom (amu)
› grams per mole (g/mol)
 Usually
rounded to 2 decimal places
1
mol of carbon
12.01 g/mol
1
mole of aluminum
26.98 g/mol
1
mole of zinc
65.39 g/mol

water
 H2O
 2(1.01) + 16.00 = 18.02 g/mol

sodium chloride
 NaCl
 22.99 + 35.45 = 58.44 g/mol

sodium bicarbonate
• NaHCO3
• 22.99 + 1.01 + 12.01 + 3(16.00)

sucrose
= 84.01 g/mol
• C12H22O11
• 12(12.01) + 22(1.01) + 11(16.00)
= 342.34 g/mol


Is the same as Average Atomic Mass
The percent of each isotope of an
element is used to find the average mass
of that element.
(mass A1 x % of A1) + (mass A2 x % of A2)
100
= Average Mass of A
This was done earlier in the year.
Remember the Skittlesium Lab?

Calculate the average atomic mass of
rubidium if 72.17% of its atoms have a
mass of 84.91 amu and 27.83% of its
atoms have a mass of 86.91 amu
(84.91 x 72.17%) + (86.91 x 27.83%) =
85.47 amu
 the
percentage by mass of
each element in a compound
mass of element
% composition 
 100
total mass
Percentage Composition
 Find
the % composition of Cu2S.
%Cu =
%S =
127.10 g Cu
159.17 g Cu2S
32.07 g S
159.17 g Cu2S
 100 =
79.852% Cu
 100 =
20.15% S
Percentage Composition
 Find
the percentage
composition of a sample that is
28 g Fe and 8.0 g O.
%Fe =
%O =
28 g
 100 = 78% Fe
36 g
8.0 g
36 g
 100 = 22% O
Percentage Composition
 How
many grams of copper are
in a 38.0-gram sample of Cu2S?
Cu2S is 79.852% Cu
(38.0 g Cu2S)(0.79852) = 30.3 g Cu
Percentage Composition
 Find
the mass percentage of
water in calcium chloride
dihydrate, CaCl2•2H2O?
%H2O =
36.04 g
 100 = 24.51%
H2O
147.02 g
 Smallest
whole number ratio of
atoms in a compound
C2H6
reduce subscripts
CH3
1. Find mass of each element. If given
percent, just say it’s a mass.
2. Find moles of each element.
3. Divide all moles by the smallest mole
to find subscripts.
4. When necessary, multiply subscripts
to get whole #’s.
If number ends in…
≈ 0.50
≈ 0.33
≈ 0.25
multiply by…
2
3
4
 Find
the empirical formula for a
sample of 25.9% N and 74.1% O.
25.9 g 1 mol
14.01 g
74.1 g 1 mol
16.00 g
= 1.85 mol N
1.85 mol
= 4.63 mol O
1.85 mol
=1N
= 2.5 O
N1O2.5
Need to make the subscripts whole
numbers  multiply by 2
N2O5
 “True
Formula” - the actual
number of atoms in a compound
empirical
formula
CH3
?
molecular
formula
C2H6
1. Find the empirical formula (EF).
2. Find the empirical formula mass.
3. Divide the molecular formula (MF)
mass by the empirical mass.
4. Multiply each subscript of the
empirical formula by the answer
from step 3.
MF mass
n
EF mass
EF n
 The
empirical formula for ethylene is
CH2. Find the molecular formula if the
molecular mass is
28.1 g/mol?
empirical formula mass = 14.03 g/mol
28.1 g/mol
= 2.00
14.03 g/mol
(CH2)2  C2H4