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Transcript
```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
1800s.
6.02 x 1023 is Avogadro’s number
7
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
13
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
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
15
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
19
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
22
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
23
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