Chapter 6 Notes
Chemical Composition
Counting by Weighing
If we know the average
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?
Average Atomic Mass
The
average of all isotopes of an element
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
What is the mass of 62 carbon atoms?
atomic mass
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
ratio of the
the two samples contain the same number of items
1 Coin
= 5.67g/ 2.27g=
15 Coins
masses, then
100 Coins
=15x5.67g
=
=100x5.67g
=
=15x2.27g
=
=100x2.27g
=
= 85.1g /34.1g=
note that the ratio of masses in each sample is
number of coins
= 567g /227g=
because each has the same
If we have 12.011g of carbon and 15.999g of oxygen, what do we know about
the number of particles of each?
The average mass of one carbon atom is
The average mass of one oxygen atom is
The ratio of the masses is the  same, what does this tell us  about the number
of particles? (remember the coins?)
Additional Notes:
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?
How many moles do you have if you have 4.55x1022 formula units of NaCl?
Molar Mass
The
of 1
of a substance
The average atomic mass given on the periodic table is the
element in grams per mole
mass of the
Carbon has a molar mass of
Chlorine has a molar mass of
To calculate the molar mass of a compound add up the average atomic masses
of each atom
CH4 would have a molar mass of
=
SO2 would have a molar mass of what?
Sodium Carbonate, Na2CO3, would have a molar mass of what?
pg 186 Example 6.5, Exercise 6.5
Additional Notes:
Homework: pg 210 (1, 2, 4, 5, 9, 19)
Mole Conversions
↔
↔
↑
↑
What mass is contained in 5.50 moles of Ne?
How many Carbon atoms are found in 43.5g of Carbon?
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?
Homework: Worksheet
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
Percent Composition
A
is always found the same way: it is always
x100
percent composition describes the percent of each element by
compound
Percent = mass of 1 mole
mass of 1 mole
in a
x 100%
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 =
%C
x 100 =
17
%H
x 100 =
%O
How many molecules of cinnamic alcohol are contained in a sample of mass
0.469 g?
Additional Notes:
Find the percent composition of
1) calcium hydroxide
Ca=
; O=
; H=
2) water
H=
; O=
3) C6H12O6
C=
; H=
; O=
4) C14H20N2SO4
C=
; H=
; N=
; S=
; O=
Homework: pg 211 (28, 29)
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?
NH4O
NH4NO
NH4NO2
↓
↓
↓
Percent composition→mass composition→ mole composition→
mole ratio→empirical formula
↑
Only round if you are within 0.1 of a whole number
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 75.2% C and 24.8% H. What is the
empirical formula of the gas?
If you had a 100g sample then you would have
C and
H
75.2g C is how many moles?
24.8g H is how many moles?
Find the ratio of the moles
There are about
empirical formula is
times more moles of hydrogen than carbon so the
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.
Homework: pg 212 (35, 37, 39, 41, 42)
Homework 2: Worksheet
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
mass by the molecular mass of the
formula
You must be given the actual
Additional Notes:
mass
Acetylene has an empirical formula of CH. The experimentally determined
molar mass is 26.04. What is the molar mass of acetylene?
=
The empirical formula then gets multiplied by
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 2
Example 6.15 pg 206
Exercise 6.15 pg 207
Homework: pg 212 (35, 37, 39, 41, 42)
Homework2: Worksheet