Download The Mole - Piscataway High School

Chemical Quantities
Chapter 10
1
How to measure matter?
Three ways to measure matter
1.
2.
3.
By counting
By mass
By volume
2
Example of 3 ways to
measure matter
By counting:

1 dozen apples = 12 apples
By mass

1 dozen apples = 2.0 kg apples
By volume

1 dozen apples = 0.2 bushels apples
3
What is Dimensional
Analysis?
A tool for solving conversion problems

To convert measurement into different
units.
Take given value and multiply it by
conversion factor
Example: 1 kg = ?? g
1 kg x 1000g = 1000g
1kg
4
Finding mass from count
What is the mass of 90 average sized
apples if 1 dozen apples has a mass of
2.0 kg?
Information:
 12 apples = 1 dozen
 1 dozen = 2.0 Kg
 Mass of 90 apples = ?

1 dozen apples 2.0 kg apples
mass of apples  90 apples 

 15kg apples
12 apples
1 dozen apples
5
What is a representative
particle?
Representative particles – refers to the
species present in a substance

Atoms, molecules, or formula unit
Substance
Element
Diatomic molecule
Covalent Compound
Ionic compound
Ion
Representative particle
atom
Molecule
Molecule
Formula unit
Ion
6
What is a mole?
Mole – (mol) amount of substance that has
the same mass in grams as its atomic or
molecular mass in atomic mass units
Mole = 6.02 x 1023 representative particles of
a substance



SI unit for measuring the amount of a substance
Developed by Amadeo Avogadro in the early
1800s.
6.02 x 1023 is Avogadro’s number
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8
Convert number of atom to
moles
How many moles of magnesium is 1.25 x 1023
atom of magnesium?
1 mol of Mg
23
moles  1.25 10 atoms of Mg 
23
6.02 x 10 atoms of Mg
0.208 mol Mg
9
Converting Moles to number
of particles
Two steps:
1.
Convert number of moles of compound to the
number of representative particle (molecule or
formula unit)
2.
Multiply the number of representative particles
by the number of atoms (6.02 x 1023)
10
Example
How many molecules are in 2.12 mol of
propane (C3H8)?
Number of moles = 2.12 mol C3H8
1 mol C3H8 = 6.02 x 1023 molecules C3H8
6.02 1023 molecule C3H8
2.12 mol C3H8 
 1.28 1024 molecules C3H8
1 moles C3H8
11
Review
What does the atomic mass mean?

The average mass of all the naturally occurring
isotopes of that element
Where do you find it on the periodic table?
What are the units for atomic mass?

Atomic mass units
12
Molar mass
Atomic mass of an element expressed
in grams is the mass of a mole of the
element
Molar mass – mass of one mole of a
substance
Element
Atomic mass
(amu)
Molar mass
(g)
Moles
(mol)
H
1
1
1
C
12
12
1
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Molecular Mass
Used for diatomic molecules
Molecule
Molecular mass
(amu)
Molar mass
(g)
Moles
(mol)
H2
2 x 1amu =
2amu
2 x 19amu =
38amu
2 x 1g = 2g
1
2 x 19g =
38g
1
2 x 35.5amu =
71 amu
2 x 35.5g =
71 g
1
F2
Cl2
14
Binary compounds and
formula mass
Formula mass – number of atoms present is
multiplied by the mass of each element.
Then the mass of the different elements are
added together.
Substance
NaCl
H2O
Al2S3
Formula mass
(amu)
1(23amu)+1(35.5amu)
= 58.5amu
2(1amu)+1(16amu) =
18amu
2(27amu)+3(32amu) =
150amu
Molar mass
(g)
1(23g)+1(35.5g)
= 58.5g
(2g)+1(16g) =
18g
2(27g)+3(32g) =
150g
Moles
(mol)
1
1
1
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Mole – mass relationship
Use the molar mass of an element or compound to
convert between the mass of a substance and the
moles of a substance
n = number of moles of a substance
Example: How many moles are in a mass of H2O =
90g?
1.
What is the molar mass?
2 x (1g) + 16g = 18g
2.
3.
1 mole of water = 18g of water
n = mass of substance / molar mass
n = 90 g / 18g = 5 moles

How many molecules are in 5 moles?

5 moles x 6.02 x 1023 molecules/mole = 3.01 x 1024 molecules
16
What is Percent
composition?
Percent composition – percent by mass of each
element in the compound
Percent by mass of an element in a compound is the
number of grams of the element divided by the mass
in grams of the compound multiplied by 100%
Equation: Percent composition from Chemical formula

Assuming 1 mole of compound
% mass = mass of element in 1 mol
Molar mass of compound
x 100%
17
Example
% mass = mass of element in 1 mole x 100%
Molar mass of compound
What is the % composition of Hydrogen in 1
mole of water?

H2O
Molar mass of H2O = 18g
 mass of 1 mol of H2 = 2g
 % mass of element = 2g x 100% = 11.1%

18g
18
Taking it a step further
1.
2.
Previous slide assumed 1 mole of
compound
How do you calculate % composition for
amounts other than 1 mole?
2 steps
Need to determine what the % composition
would be for 1 mole
Multiply this % times the amount of
substance
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Example
What is the % composition of Hydrogen
in 50 g of water?

Step 1: H2O
Molar mass of H2O = 18g
 mass of 1 mol of H2 = 2g
 % mass of element = 2g x 100% = 11.1%

18g

Step 2: Multiply percentage by amount

(11.1/100) x 50g = 5.55 g of H2
20
How to determine # of moles
from chemical equation?
Coefficients in front of molecule or element indicate
how many moles are present.
Example: 2 H2 + O2 2H2O
How many moles of H2?

2 moles of H2
How many moles of O2?

1 mole of O2
How many moles of H2O?

2 moles of H2O
21
Proof of conservation of
mass
2 H2 + O2 2H2O
Calculate the molar mass for each side of this
equation.
Left side


H2 = 2g, O2 = 2(16g)=32 g
 total = 2(2g) + 1(32 g) = 36 g
Right side


H2O = 2(1g) + 16g = 18g
 total = 2(18g) = 36 g
Same amount of mass on each side even
though not same number of moles
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What do balanced chemical
equations tell you?
1.
2.
3.
4.
5.
6.
7.
Number and type of substances needed to
cause a particular reaction
Number and types of substances produced
in a particular reaction
# of moles of each reactant needed.
# of moles of each product produced.
Molar mass of each reactant needed.
Molar mass of each product formed.
Physical state of each of the reactants and
products
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