Measurements of Matter
•Mass, volume, length, density… all
these are measurements of matter.
•What is matter made of though? atoms
•If you measured the density of an object
to be 2.04 g/mL, you have a certain
number of atoms all joined together in a
certain amount of space.
•So how many atoms are in there?
Why is it important to know HOW MANY
atoms there are in a substance?
Chemistry is just like cooking!!! If you are making
cookies and your recipe calls for 3 eggs in order to
make 24 cookies, what happens if you only have
1 egg? Will you make 24 cookies?
No, quantity matters 
Same would be true if you were making table salt
(NaCl), except it’s a lot more dangerous to make.
https://www.youtube.com/watch?v=d2geiGKFveE
The MOLE
It’s a beauty mark…
It’s a small furry garden pest…
No, wait… its how we count ATOMS!
What is a Mole and how
is it used in Chemistry?
• Problem: How do you count something as
small as an atom?
• Solution: The Mole
• A mole is a quantity (certain amount) of
atoms. Due to atoms being so tiny, they are
hard to individually measure. Scientists
have to measure atoms in large quantities.
The Mole
•Specific quantity (amount) of
atoms in a substance. Basically,
a word that represents a
specific number (ex: pair = 2)
•1 mole = 6.02 x 1023
representative particles
(a lot of very tiny particles)
•This value is called Avogadro’s
Number
What is a mole?
Try this
What are representative particles?
•smallest particle that retains chemical and
physical properties
•3 types depending on the compound:
•Atoms: Single element
•Molecules: covalent compound
•Formula Units: Ionic Compounds or ions
How do we use it?
•Used to count groups of small
particles for the purpose of
mixing them with other particles
in the lab.
•Ex: If 2.5 moles of sodium were
used in an experiment, how many
atoms of sodium were there?
1 mole = 6.02x1023
2.5 mole = ___________
Conversions
Convert: change units (cancel Unit) by using a
conversion factor
Conversion Factor: equality using different units
Ex:
19
Exactly 9.673 x 10 molecules
of water are needed to safely
react in with sodium. How
many moles of water are
needed?
The Mole
•In a lab, the exact number of
particles are needed before you
can mix them with anything else.
•Problem – No way to count the
particles (too small) and no piece
of equipment that measures in
moles. How do you measure a
mole?
•Solution – Using a balance to
measure the mass of a mole
(molar mass)
Molar Mass
The mass of 1 mole of
any substance.
The periodic table contains the molar
mass of all elements
Molar Mass = mass of 1 mole (6.02x1023)of its atoms.
6.02x1023 atoms Carbon = 1 mole carbon = 12.01 grams Carbon
FORMULAS - review
MgCl2
•The subscript is the
number at the bottom of
a formula.
•There is 1- Mg & 2 - Cl
How to calculate molar mass
1. Identify the # of atoms of each
element
2. Multiply # atoms by the atomic
mass of that element. (round to 2
#’s after the decimal)
3. Add them all together
4. Grams (g) is the unit
Using Molar Mass as a conversion factor
The periodic table contains the molar
mass of all elements.
Molar Mass = mass of 1 mole (6.02x1023)of its atoms.
6.02x1023 atoms Carbon = 1 mole carbon = 12.01 grams Carbon
• If 2.5 moles of carbon were used in an experiment, how many
grams of carbon must you weigh out?
Using molar mass
• How many grams are in 15.7 mole MgCl2
The Mole of a gas
Why would it be difficult to
measure the mass of helium?
(how would it stay on the balance)
Scientist have discovered that 1 mole of
any gas has a constant volume at STP.
(standard temperature & pressure)
1 mole = 22.4 L of any gas
This value is called Molar Volume
Using molar volume as a conversion factor
•How many moles are in 12.95 L of oxygen gas?
• How many liters are in 0.758 moles of nitrogen gas?
What does a mole tell us?
1. Number of particles – Avagadro’s
number
2. Mass particles in solid or liquid –
Molar mass
3. Volume of particles in a gas –
Molar Volume
What is a percentage mean?
•How is a percent calculated?
•What is the percentage of girls in the
class?
•What is the percentage of students in
Mrs. Johnson’s class if she has 10 girls
and 12 boys?
Percent Composition
• The percent BY MASS of each element in a
compound – divide the element’s total mass (part)
by the molar mass (whole) then multiple by 100 to
get the percent.
(PART)
(WHOLE)
• Ex: % composition of MgCl2
Mg – 1 (24.31) =
Cl – 2 (35.45) =
(PART)
(WHOLE)
24.31 / 95.21 x 100 = 25.53% Mg
70.90 / 95.21 x 100 = 74.47% Cl
Molar mass = 95.21 g/mol
Practice - Calculate the % comp of KMnO4:
K – 1 (39.10) =
39.10 / 158.04 x 100 =
24.74% K
Mn – 1 (54.94) = 54.94 / 158.04 x 100 =
34.76% Mn
O – 4 (16.00) = 64.00 / 158.04 x 100 =
40.50% O
molar mass KMnO4 = 158.04
How do we use % composition
in the lab?
•Knowing the % composition of an element
allows you to find the mass of an element
in the compound.
•How many grams of carbon are in
Calculating the mass of an element in a sample
1. Find the % comp of the element in the
compound
2. Change the % to a decimal (move decimal 2
times to the left or divide by 100)
3. Multiply that decimal by the amount (g) of the
sample.
Ex: Calculate the mass of chlorine in 203.5 grams
of MgCl2. (use the % we found earlier)
74.47% Cl = .7447 x 203.5 = 151.5 grams Cl
Practice:
Calculate the amount of oxygen in 15.75 grams of water.
H – 2(1.01) = 2.02
O – 1(16.00) = 16.00 / 18.02 x100 =
Molar mass H2O = 18.02
88.79% O
88.79% O = .8879 x 15.75 = 13.98 grams O
Complete % comp worksheet
Identifying an
unknown substance
Empirical & Molecular
Formulas
Mass Spectroscopy
• Video
• Video
• Video
• Video
• Video
• video
Empirical Formula
The lowest whole
number ratio
(subscripts) of elements
in a compound.
Cannot be reduced!!!
not empirical
empirical
Ex: C6H12O6 
CH2O
Molecular Formula
Actual number of atoms in a chemical compound
molecular
EX: C12H24O12
Molecular Formulas can be reduced to Empirical Formulas
molecular
EX: C12H24O12  CH2O
empirical
Different molecular formulas can have similar empirical formulas
EX:
molecular
N3O9 
N12O36 
empirical
NO3
PRACTICE:
1. Identify each as empirical (can’t be reduced) or
molecular (can be reduced)
2. If its molecular – write the empirical
1.
2.
3.
4.
5.
C2H4 molecular
NO3 empirical
S9Cl12 molecular
C3Cl9 molecular
N4S9 empirical
CH2
S3Cl4
- empirical
- empirical
CCl3 - empirical
Finding Empirical Formula from Percent Composition
• Ex: A compound was found to be 54.53% Carbon, 9.15%
Hydrogen, and 36.32% Oxygen. Find its Empirical Formula.
Steps:
1. Assume a 100g sample (change %  g)
2. ÷ by the molar mass of that element to find moles (sig fig it!)
3. Identify the lowest # of moles and ÷ them all by that number
4. Round each to the nearest whole # (0.0 or 0.9)
5. The resulting whole #are the subscripts for that element in the
empirical formula
Calculating Empirical Formula
63.5% Silver
8.2% Nitrogen
28.3% Oxygen
63.5 g Ag
107.87
8.2 g N
14.01
28.3 g O
16.00
.589 mole Ag
.589
.59 mole N
.589
1.77 mole O
.589
1
1
AgNO3
3
Calculating Empirical Formula (special)
60.00%C
60.00g C
12.01
4.48%H
4.48g H
1.01
35.53%O
35.53g O
16.00
4.996 mole C
2.221
4.44 mole H
2.221
2.221 mole O
2.221
2.249
2
1
x4
x4
x4
9
8
C9H8O4
4
Calculating Molecular Formula
1.Find the empirical formula
2.Calculate the molar mass of your
empirical formula
3.Identify the molar mass of your molecular
(GIVEN in the problem every time!)
4.Divide the molecular mass / empirical
mass
5.Round to the nearest whole #
6.Multiply the whole # by the subscripts in
the Empirical formula
Practice
If a compound has an empirical formula of
NO3 and a molecular mass of 186g – what is
the molecular formula?
Empirical formula: NO3
Molecular mass (given)
empirical mass
3 x NO3
=
N3O9
molar mass:
62.01g
186g
62.01
Calculating Empirical Formula (special)
60.00%C
60.00g C
12.01
4.48%H
4.48g H
1.01
35.53%O
35.53g O
16.00
4.996 mole C
2.221
4.44 mole H
2.221
2.221 mole N
2.221
2.249
2
1
x4
x4
x4
8
4
9
C9H8O4