It’s a beauty mark…
It’s a small furry garden pest…
No, wait… its how we count ATOMS!
The mass of 1 mole of
a substance.
Calculate the molar mass
Calculate the molar mass of
1 mole of magnesium
chloride. (first you need a formula)
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
Molar Mass
MgCl2
Mg – 1 (24.31) = 24.31
Cl – 2 (35.45) = 70.90
1 mole MgCl2 = 95.21 g MgCl2
or
95.21 g/mol
Molar Mass
•Molar mass allows scientist a way to
measure moles in a lab.
• Molar mass of Fe =
• Molar mass of O2 =
• Molar mass of Cu(OH)2=
Calculate the molar mass:
(you MUST write the formula correctly before answering)
1 mole of Ammonium phosphide
1 mole of Trinitrogen pentachloride
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
(PART)
(WHOLE)
Mg – 1 (24.31) = 24.31 / 95.21 x 100 = 25.53% Mg
Cl – 2 (35.45) = 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
Calculating the amount 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 amount 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 =
88.79% O
Molar mass H2O = 18.02
88.79% O = .8879 x 15.75 = 13.98 grams O
Complete % comp worksheet
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
empirical
• EX: C12H24O12  CH2O
• Different molecular formulas can have
similar empirical formulas
•
molecular
• EX: N3O9
Molecular formula: C76H52O46
empirical

N12O36 
NO3
Empirical formula: ___________
PRACTICE:
1. Identify each as empirical (can’t be reduced) or
molecular (can be reduced)
2. If its molecular – write the empirical
CH2
1. C2H4
molecular
2. NO3
empirical
3. S9Cl12
molecular
S3Cl4 - empirical
4. C3Cl9
molecular
CCl3 - empirical
5. N4S9
empirical
- 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:
• Assume a 100g sample (change %  g)
• Use molar mass to find moles of each
• Divide all moles by the smallest number of moles
• Round each to the nearest whole # (sometimes you have to
multiply to get a whole number - special)
• 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
8.2 g N
28.3 g O
107.87
14.01
16.00
.589 mole Ag
.589
1
.59 mole N
1.77 mole O
.589
.589
1
AgNO3
3
Calculating Empirical Formula (special)
60.00%C
4.48%H
35.53%O
60.00g C
4.48g H
35.53g O
1.01
16.00
12.01
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
C9 H 8O4
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 everytime!)
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 NO 3 and a
molecular mass of 186g – what is the molecular formula?
Empirical formula: NO 3
molar mass: 62.01g
Molecular mass (given)
186g
empirical mass
3 x NO3
62.01
=
N 3O 9
What is a mole?
• For counting matter
(quantity) in chemistry we
use the mole
• Moles are used to
correctly measure
chemicals for reactions in
a lab.
• Problem – no way to
“physically” measure a
mole. (no lab equipment
measures moles)
• Solution – molar mass
(balance)
Using Molar Mass
Remember – Molar mass is the mass (grams) of 1 mole
1 mole Fe = _________grams Fe
2.5 mole Fe = ________ grams Fe
113.5 grams Fe = _______ moles Fe
Mass to mole = divide by molar mass
Mole to mass = moletiply by mole mass 
Using molar mass
How many grams are in 15.7 mole MgCl 2
How many moles are in 0.75 grams of silver?
What is the mass of 30.7 mole water?
Using molar mass
In the lab, Mrs. Mathieson needs 2.57 moles of NaCl to do an
experiment. How many grams would be needed to equal
2.57 moles of NaCl?
After doing the experiment, Mrs. Mathieson has 1.02 moles
of NaCl remaining – how many grams does that equal?
What is a mole?
•For counting matter
(quantity) in chemistry
we use the mole
•1 mole = 6.02 x 1023
representative particles
(particles are very tiny)
•This value is called
Avogadro’s Number
What are REPRESENTATIVE PARTICLES?
• ie. the 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
Using Avagadro’s Number
Remember there are 6.02 x 1023 particles (atoms, molecule, f.u) in 1 mole
How many atoms are in 1 mole?
How many atoms are in 2.10 moles of Copper?
How many moles are in 4.21 x 10 26 atoms of aluminum?
What is a mole?
•For counting matter
(quantity) in chemistry
we use the mole
•1 mole = 22.4 L of any
gas
•This value is called
Molar Volume
Using molar volume
How many liters are in 1 mole of any gas?
How many liters are in 12.95 L of oxygen gas?
How many moles are in 0.758 moles of nitrogen gas?