Chapter 8 Notes
Chemical Composition
Counting by Weighing
If we know the
mass of one atom, then we can use the mass to count atoms
  If we know the average mass  of a marble is 10.4 grams,  then if we mass out 187g of
marbles, how many do we  have?
o
Average Atomic Mass
 The
average of all isotopes of an element
o What is the difference between atomic  mass and mass number?

(atomic mass unit)—1/12 the mass of  a carbon-12 atom
 The mass given on the periodic table is the
atomic mass
o What is the mass of 62 carbon atoms?
62 carbon atoms
Example 6.2 pg 177
Practice Problem 6.1, 6.2
Additional Notes:
Counting Atoms
If we then know the average mass of an atom, we can count atoms in a sample
The average mass of a carbon atom is 12.01amu. Therefore if we have 1.45x1019 carbon atoms,
what mass would the sample have?
_____
If we have a mass of 6.43x107amu of carbon, how many atoms do we have?
Additional Notes:
If the ratio of masses of two samples
contain the same number of items
1 Coin
the ratio of the
15 Coins
= 5.67g =
2.27g 1
masses, then the two samples
100 Coins
=5.67g
=15x5.67g
=
=100x5.67g
=
=2.27g
=15x2.27g
=
=100x2.27g
=
= 85.1g =
34.1g 1
note that the ratio of masses in each sample is
of coins
= 85.1g =
34.1g 1
because each has the same number
Additional Notes:
If we have 12.011g of carbon and 15.999g of oxygen, what do we know about the number of
atoms in each sample?
  The average mass of one carbon atom is
  The average mass of one oxygen atom is
  The
of the masses is the  same, what does this tell us  about the number of
particles? (remember the coins?)
Moles
Used to measure very small objects such as atoms or molecules
  It is simply a
. You can attach any  object to it (atoms, molecules, formula units,
etc.)
Avogadro's Number (N) =
units in 1 mole
  1 mole of NH3 contains
molecules of NH3
  1 mole of Fe contains
atoms of iron
 How many atoms are in 7.70 moles of Nickel?
o

How many moles do you have if you have 4.55x1022 formula units of NaCl?
o
Additional Notes:
Molar Mass
The
of 1

of a substance
The average atomic mass given on the periodic table is the
grams per mole

Carbon has a molar mass of

Chlorine has a molar mass of
mass of the element in
Molar Mass of a compound
 To calculate the molar mass of a compound add up the average atomic masses of each
atom
o  CH4 would have a molar mass of
o
=
SO2 would have a molar mass of what?
o
Sodium Carbonate would have a molar mass of what?
Additional Notes:
pg 186 Example 6.5, Exercise 6.5
Homework: pg 239 (1, 3, 4, 5, 7, 25, 27)
Mole Conversions
What mass is contained in 5.50 moles of Ne?
5.50mol
How many Carbon atoms are found in 43.5g of Carbon?
43.5gC
What is the mass of 2.11x1024 copper atoms?
What is the mass of 3.01x1023  formula units of sodium chloride?
How many atoms are in 54.3g of Cobalt?
Additional Notes:
Homework: pg 240 (9, 11, 17, 24, 39)
Practice
How many moles of Copper (II) nitrate would be found in 64.9g?
How many molecules of oxygen gas would be in 25.99g?
How many grams of carbon dioxide would be found in 4.2x1014 molecules?
Homework: Worksheet
Additional Notes:
Percent Composition
A
is always found the same way: it is always
x100
percent composition describes the percent of each element by
Percent =
x 100%
in a compound
Additional Notes:
Cinnamic alcohol is used mainly in perfumery, particularly in soaps and cosmetics. Its
molecular formula is C9H10O. Calculate the percent composition by mass of C, H, and O
in cinnamic alcohol.
x100 =
x100 =
x 100 =
How many molecules of cinnamic alcohol are contained in a sample of mass 0.469 g?
Find the percent composition of
1) calcium hydroxide
Ca=
; O=
2) water
H=
; O=
3) C6H12O6
C=
; H=
4) C14H20N2SO4
C=
; H=
; H=
%; O=
; N=
;   S=
; O=
Additional Notes:
Homework: pg 242 (40, 43-45, 52)
Empirical Formula
formula—the simplest

among atoms in a compound
the subscripts

C6H12O6 would reduce to

C8H18 would have what empirical formula?

NH2O is the empirical formula for which of these?
o
o
o
NH4O
NH4NO
NH4NO2
Additional Notes:
Percent composition→mass composition→ mole composition→mole ratio→empirical formula
If you are given the percent composition, you can find the empirical formula of a compound

A sample of methane gas is found to be 75.1% C and 24.9% H. What is the empirical
formula of the gas?
o
If you had a 100g sample then you would have
o
75.1g C is how many moles?
o
24.9g H is how many moles?
and

Find the ratio of the moles

There are about 4 times more moles of hydrogen than carbon so the empirical formula is
Additional Notes:
Stannous fluoride is added to toothpaste to help prevent tooth decay. What is the empirical
formula if the compound is 24.25% F and 75.75% Sn, by mass?
What is the empirical formula of a compound that contains 0.483g of N and 1.104 g of O?
Vitamin C contains three elements: carbon, hydrogen, and oxygen. Analysis of pure vitamin C
indicates that the elements are present in the following mass percentages:
C = 40.9%
H = 4.60%
O = 54.5%
Use the data to determine the simplest formula for vitamin C.
Additional Notes:
Homework: pg 243 (55, 57-61)
Iron reacts with oxygen to form 3 compounds: FeO, Fe2O3, and Fe3O4. One of these,
magnetite, is 72.36% Fe and 27.64% O by mass. What is the formula of magnetite?
Molecular Formula
To determine the molecular formula (actual formula) from the empirical formula, divide the given
molecular mass by the
mass of the
formula

You must be given the actual
mass
Acetylene has an empirical formula of CH. The experimentally determined molar mass is
26.04. What is the molecular formula of acetylene?
=
The empirical formula then gets multiplied by 2
Additional Notes:
Caffeine has the following percent composition: carbon 49.48%, hydrogen 5.19%, oxygen
16.48% and nitrogen 28.85%. Its molecular weight is 194.19 g/mol. What is its molecular
formula?
First find the empirical formula
Find the mass of the empirical formula
Divide the molecular mass by the empirical mass
Multiply each subscript by
Additional Notes:
Homework: Worksheet