What does the little number below
each
Element mean?
What are its’ units?
amu/atom
What is an amu
one amu is defined as
th
1/12
of
12C
Average atomic mass or atomic
weight is determined by a mass
spectroscope.
example: Analysis of a sample of
carbon was found to have the
following isotopes
12C – 11.999 97.3%
13C – 12.998 2.6%
14C – 13.966 0.10%
What is the atomic weight of Carbon
12C
– (11.999)
13C – (12.998)
14C – (13.966)
(.973) = 11.675
(.026) = .33795
(.0010) = .013966
12.0269
How much is a mole?
How high could you
stack a mole of sheets
of paper?
How much would a mole of iron weigh?
Could you lift it?
Look on the back inside cover for amu and mole
6.02214 x 1023 55.85 amu 1.66054 x 10-24 g*
=55.85
g
atoms of Fe 1atom of Fe
1 amu
What is the Mole?
One mole is equal to the number of
carbon atoms in exactly 12.00 grams of
23
C-12: 6.02 x 10 atoms
# of atoms or molecules
6.02 x 10 23
atoms or molecules/mole
Mole
x g/ mole
Mass of atom
or molecule
22.4 L / mole
volume of gas
The Shopping Mole
• Calculate the numbers of atoms
of cesium found in 3.57 moles of
cesium
• Calculate the numbers of atoms
of cesium found in 3.57grams of
cesium
• Calculate the mass of 4.82 x 1023
atoms of cesium
Calculating the molar mass of a compound (NH4)2S
1. Calculate the mass of 4.65 moles of
(NH4)2S
2. Calculate the numbers of atoms of
nitrogen found in 3.57grams (NH4)2S
3. Calculate the mass of nitrogen in 3.76 x
1024 formula units of (NH4)2S
Determining Percent Composition of Compounds
Given the compound: Al(HPO3)3
What are the percent compositions of each of the atoms
from which the above compound is constructed?
Mass of Aluminum= 1 mol Al
26.9 grams
mole of Al
Mass of hydrogen = 3 mols H
Mass of Phosphorus = 3 mols P
1.01 gram
mole of H
30.9 gram
mole of P
Mass of oxygen = 9 mols O
16.0 gram
mole of P
= 26.9g Al
= 3.03 H
266 g
= 92.7 g P
= 144 g O
Al(HPO3)3
26.9 g
mass of Al
Mass percent of Al:
=
10.1%
=
1.13%
=
34.8%
= 144 g =
x 100
266 g
mass of Al(HPO3)3
54.1%
mass of Al(HPO3)3
x 100 =
mass of H
Mass percent of H:
mass of Al(HPO3)3
x 100 =
mass of P
Mass percent of P:
mass of Al(HPO3)3
mass of O
Mass percent of O:
x 100
266 g
3.03 g
266 g
92.7 g
=
266 g
Empirical formula
A compound is composed of
Cl 71.65% and C 24.27% and H
4.07% what is the empirical
formula? The Molar mass is
98.96g/mole, what is the
molecular formula?
Determining the Empirical Formula of a
Compound Using Combustion Analysis
Combustion Analysis
0.255 g isopropyl alcohol is combusted
mass difference of H2O absorber = 0.306 g
mass difference of CO2 absorber = 0.561 g
mass difference of H2O absorber = 0.306 g
mass difference of CO2 absorber = 0.561 g
grams of C: 0.561 g CO2
grams of H: 0.306 g H2O
1 mole CO2
12 g C
44 g CO2
1 mole CO2
1 mole H2O
18 g H2O
2.02 g H
1 mole H2O
= 0.153g C
= 0.0343 g H
grams of O: 0.255 g - 0.153 g - 0.0343 g H = 0.0677 g
C: 0.153 g C
1 mol C
12 g C
H: 0.0343 g H
1 mol H
1.01 g H
O: 0.0677 g O
1 mol O
16 g O
= 0.0128 mol C
= 0.0340 mol C
1
0.0043
1
0.0043
1
= 0.0043 mol O
0.0043
= 2.97
= 7.91
= 1.00
Assume that the empirical formula for the
compound is C3H8O
Ascorbic acid (Vitamin C) contains 40.92 percent
C, 4.58 percent H, and 54.50 percent O by mass.
What is the empirical formula of ascorbic acid?
Ethylene glycol, the substance used in automobile
antifreeze, is composed of 48.7 % C, 9.7 percent H,
and 51.6 percent O by mass. Its molar mass is
62.1 g/mol. (a) What is the empirical formula of
ethylene glycol? (b) What is the molecular formula
So What is Stoichiometry?
mass of A in g mole 3(moles C) C g = mass C
A g 1(moles A) mole
1A + 2B → 3C + 4D
Na2SO3(aq) + 2AgNO3(aq)  Ag2SO3(s) + 2NaNO3(aq)
If 3.4 g of silver nitrate is mixed w/
excess sodium sulfite, what would
be the theoretical yield for silver
sulfite?
a) in moles
b) in formula units
c) in L at STP
d) in grams
Na2SO3(aq) + 2AgNO3(aq)  Ag2SO3(s) + 2NaNO3(aq)
• If 3.4 g of silver nitrate is mixed
sodium sulfite, how many
grams of sodium sulfite would
you need to react completely?
Na2SO3(aq) + 2AgNO3(aq)  Ag2SO3(s) + 2NaNO3(aq)
If 4.5 g of sodium sulfite is
mixed w/ excess silver
nitrate, what would be the
theoretical yield for silver
sulfite?
What would be the percent
yield if 4.1g was produced in
the lab?
Na2SO3(aq) + 2AgNO3(aq)  Ag2SO3(s) + 2NaNO3(aq)
If 4.3 g of silver nitrate is mixed
w/ 3.2g sodium sulfite, what
would be the theoretical yield
for silver sulfite?
How much of the excess reagent
is left over?
Using Stoichiometry in the Laboratory
Na2SO3(aq) + 2AgNO3(aq)  Ag2SO3(s) + 2NaNO3(aq)
20 ml
20 ml
0.12 M
0.35 M
1. How many grams of silver sulfite would be
produced?
2. What would be the molarity of the sodium
nitrate?