Transcript The Mole Concept

Title: Lesson 3 The Mole Concept
Learning Objectives:
• Understand that the mole is a fixed number of particles and refers to the
amount, n, of substance.
• Understand that masses of atoms are compared on a scale relative to 12C and
are expressed as relative atomic mass (Ar) and relative formula/molecular mass
(Mr).
• Calculate numbers of particles using Avogadro’s number
Lesson 2: Moles and Molar Mass
• Objectives:
– Calculate the relative mass and molar mass of
substances
– Relate the mass of a substance to a quantity in
moles
– Conduct an experiment to determine the number
of moles of water of crystallisation
Different types of mass
• Relative atomic mass, Ar:
Used when referring to the mixture of naturally occurring
isotopes in an element and is found in the periodic table
Definition
• The weighted mean mass of an atom of an element
compared with 1/12 of the mass of an atom of carbon-12.
• E.g. The weighted average of silver is 107.87. This means
on average silver has 107.87 times the mass of 1/12th of a
carbon atom.
• NOTE: The are no units as quantities are relative.
Different types of mass
• Relative molecular mass Mr:
Only used when referring to bonded molecules. E.g. Cl2, H2O.
e.g. H2O has a mass of 18:
H=1 x2=2
O = 16 x 1 = 16
Definition
• The weighted mean mass of an atom of a molecule compared
with 1/12 of the mass of an atom of carbon-12.
• NOTE: The are no units as quantities are relative.
If you compare masses with each other in relative terms you can
see the relationship:
• Oxygen has a mass
16 times greater
than an atom of
Hydrogen
• Sulphur has a mass
32 times greater
than an atom of
Hydrogen
• Sulphur has a mass
2 times greater than
Oxygen
Calculating Mr
• HCl


– Ar(H) = 1.01
– Ar (Cl) = 35.45


– Mr = 1.01 + 35.45 = 36.46
• C2H4
– Ar(C) = 12.01
– Ar (H) = 1.01
– Mr
= 2x12.01 + 4x1.01
= 28.06
H2SO4


Ar(H) = 1.01
Ar (S) = 32.06
Ar(O) = 16.00
Mr = 2x1.01 + 32.06 + 4x16.00
= 98.08
Mg(OH)2




Ar(Mg) = 24.31
Ar(O) = 16.00
Ar (H) = 1.01
Mr = 24.31 + 2x16.00 + 2x1.01
= 58.33
Relative formula mass, Mr
The relative formula mass of a compound is the relative atomic
masses of all the elements in the compound added together.
E.g. water H2O:
Relative atomic mass of O = 16
Relative atomic mass of H = 1
Therefore Mr for water = 16 + (2x1) = 18
Work out Mr for the following compounds:
1) HCl
H=1, Cl=35 so Mr = 36
2) NaOH
Na=23, O=16, H=1 so Mr = 40
3) MgCl2
Mg=24, Cl=35 so Mr = 24+(2x35) = 94
4) H2SO4
H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98
5) K2CO3
K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138
04/08/2016
Calculating percentage mass
If you can work out Mr then this bit is easy…
Percentage mass (%) =
Mass of element Ar
Relative formula mass Mr
x100%
Calculate the percentage mass of magnesium in magnesium oxide, MgO:
Ar for magnesium = 24
Ar for oxygen = 16
Mr for magnesium oxide = 24 + 16 = 40
Therefore percentage mass = 24/40 x 100% = 60%
Calculate the percentage mass of the following:
1) Hydrogen in hydrochloric acid, HCl
3%
2) Potassium in potassium chloride, KCl
52%
3) Calcium in calcium chloride, CaCl2
36%
4) 04/08/2016
Oxygen in water, H2O
89%
What do the following have in
common?
A dozen
A grand
A score
A pair
A trio
A Mole
• A ‘mole’ represents a large number (just like dozen
represents 12):
• A mole is:
–
–
–
–
–
6.02 x 1023
602,000,000,000,000,000,000,000
Six hundred and two thousand quadrillion
Given the symbol, L
This number is called Avogadro’s number after the Italian
scientist Amedeo Avogadro who first proposed it
• Picture – all the grains of sand, on all the beaches and in all
the deserts in the world; that is about a tenth of a mole!
Some Calculations
•We can use this equation
to calculate a number of
moles from a number of
particles
N
n
L
•Where:
•n = quantity in moles
•N = number of
particles
•L = Avogadro’s
Constant (6.02x1023)
1.
Example 1: You have 3.01x1022 atoms of carbon. How
many moles is this?
•n = N / L
•n(C) = 3.01x1022 / 6.02x1023
•n(C) = 0.0500 mol
2.
Example 2: You have 6.02x1024 molecules of water. How
many moles of hydrogen atoms are present?
•n = N / L
•n(H) = 2 x n(H2O)
•n(H) = 2 x 6.02x1024 / 6.02x1023
•n(H) = 20.0
3.
Example 3: How many atoms of hydrogen are there in
2.5 moles of methane (CH4)?
•N(H) = 4 x N(CH4)
•N(H) = 4 x n(CH4) x L
•N(H) = 4 x 2.5 x 6.02x1023
•N(H) = 6.02x1024
The number of particles in a molecule
1 mol O2 means one mole of O2 molecules = 6.02 x1023 molecules of O2.
In a molecule of O2 = 2 atoms of Oxygen
Therefore one mole of 02 molecules is made up of 2 lots of 6.02 x 1023 Oxygen atoms.
So 1 mole of O2 molecules = 2 moles of Oxygen atoms
Table to show the relationship between number of moles of molecules and the
number of moles of particular atoms.
Compound
Moles of molecules
Moles of O atoms
H2O
0.1
0.1
SO2
0.1
0.2
SO3
0.1
0.3
H3PO4
0.1
0.4
O3
0.5
1.5
CH3COOH
0.2
0.4
N
n
L Refresh: Some Calculations
1.
How many moles of ethene
(C2H4) are there in 1.5x1022
molecules?
2.
How many moles of oxygen
atoms in 1.20x1024 molecules of
sulphuric acid (H2SO4)? How
many moles of oxygen molecules
could they make?
3.
How many molecules in 3.0
moles of nitrogen gas?
4.
How many chloride ions are
released on dissolving 0.050
moles of calcium chloride
(CaCl2)?
BONUS QUESTION: How many moles of people are there on the planet?
N
n
L Refresh: Some Calculations
1. How many moles of ethene (C2H4)
are there in 1.5x1022 molecules?
Answer:
1 mole C2H4 = 6.02x1023 molecules
1.5x1022 / 6.02x1023 =
0.025 moles
2. How many moles of oxygen atoms in
1.20x1024 molecules of sulphuric acid
(H2SO4)? How many moles of oxygen
molecules could they make?
Answer:
Moles of H2SO4 =1.20x1024 / 6.02x1023 =
1.99 moles of H2SO4
2 moles of H2SO4 = 2 x 4 = 8 moles of
Oxygen)
Can make 4 moles of O2 molecules
3. How many molecules in 3.0 moles
of nitrogen gas?
Answer:
1 mole of N2 = 6.02x1023 molecules
3 moles of N2 = 3 x 6.02x1023
molecules of N2 = 18.06x1023
4. How many moles chloride ions are
released on dissolving 0.050 moles of
calcium chloride (CaCl2)?
Answer:
0.050 moles = CaCl2
Chloride ions x 2 = 0.050 x 2 = 0.1
moles of chloride ions.
BONUS QUESTION: How many moles of people are there on the planet?
How do you determine Avogadro’s
number?
• There are a number of ways including:
– Electrolysis
– Measurement of electron mass
– X-ray crystal density
– Molecular monolayers
• You will be determining L by measuring the
area of a molecular monolayer
– Not the most accurate but feasible in our lab!
Doing the calculations
If we drop a small amount of stearic acid onto water, it will spread out to form a
monolayer and we can measure the area this occupies. The monolayer will be
circular (it is formed in a circular watch glass) so:
If we know the cross-sectional (end-on) area of a molecule of stearic acid (this is
approximately 2.1x10-15 cm2) we can calculate the number of molecules in our
monolayer:
If we know the mass of stearic acid used and the relative molecular mass we can
work out the number of moles we have:
Finally we can combine the number of molecules and the number of moles to
work out Avogadro’s constant:
Ms Easton’s Results
Data from experiment
Diameter of watch glass = 4.9cm
Monolayer = 8 drops
Mass of 8 drops = 9.630 (drops +
cylinder) – 9.443 (cylinder) = 0.187g
Molecular formula Stearic Acid =
C18H3602
Analysis
•How does your calculated value of Avogadro’s constant
compare to the literature value?
Mine was too low!
•What do you think are the main sources of error in this
experiment? How could they be overcome?
Not cleaning the watch glass properly – this affected how the
monolayer was formed. Pasteur pipettes not
accurate/calibrated correctly – drop sizes were not the same.
•What assumptions does our method make? Are these valid
assumptions?
That the monolayer was 1 layer thick, that we could accurately
measure the mass of Stearic Acid used. Not valid assumptions
because we couldn’t prove it by determining the correct value
for Avogadro’s number.
Key Points
N
n
L
particles
moles 
23
6.02 x10