KENT TRIPLE SCIENCE NETWORK
From Kent Science Resource
Centre
Formulae, Equations & Moles
[Getting to grips with quantitative chemistry]
How comfortable are
you with quantitative
chemistry?
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Personal knowledge?
Delivery to the
students?
Practical aspects?
Difficulties and misconceptions
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Students often lack secure understanding of basic chemical
concepts.
Many of the ideas involved are abstract and/or difficult to
comprehend:
• ‘amount of substance’ (or ‘chemical amount’);
• Avogadro’s number (too large to be easily ‘seen’);
• link between macroscopic and microscopic scales.
The definition of ‘mole’ often seems quite confused.
The language & vocabulary used can cause problems.
The ‘mole’ is often taught purely as an abstract mathematical
idea.
Important basic concepts …(1)
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Matter is made from tiny particles (atoms) invisible to
the naked eye.
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Chemical reactions produce new substances.
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In a chemical reaction, atoms are rearranged but no
atoms are created or destroyed. Mass is conserved.
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Chemists need to be able to accurately measure
amounts of substance in order to be able to
control/manipulate chemical reactions.
Conservation of mass (1)
Conservation of mass (2)
Conservation of mass (3)
Sealable bag with citric acid
and sodium bicarbonate.
Weighed – with water
– before and after,
Important basic concepts …(2)
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Chemical formulae – understanding and using.
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Relative atomic mass (Ar).
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Relative molecular (or formula) mass (Mr).
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Balanced symbol equations – interpreting/writing.
Understanding chemical formulae
[Important basic concepts]
Understanding chemical formulae
[Important basic concepts]
2−
3+
2+
3−
1−
1+
Understanding chemical formulae
[Important basic concepts]
Advantages/disadvantages of this model?
1−
2+
1−
Web simulations
Two from PhET
Build a molecule
http://phet.colorado.edu/en/simulation/build-a-molecule
Balancing Chemical equations
http://phet.colorado.edu/en/simulation/balancing-chemicalequations
Understanding chemical formulae
[Important basic concepts]
H2O
Advantages/disadvantages of different models?
Relative atomic mass
[Important basic concepts]
100 chlorine atoms
75 atoms are 35Cl
25 atoms are 37Cl
Total mass of all 100 atoms
= (75 x 35) + (25 x 37)
= 3550
So … the average mass of a
chlorine atom is 3550/100
= 35.5
i.e. Ar(Cl) = 35.5
Relative molecular (formula) mass
[Important basic concepts]
One model that can help some
students ‘see’ that relative
molecular mass (Mr) = sum of the
Ar values of the component
elements.
Mr(H2O)
= 1+ 1+ 16
= 18
Balanced equations
[Important basic concepts]
2H2
+
O2

2H2O
Link to a real reaction
Mix different ratios of methane and
oxygen in 500ml plastic bottles.
Which gives the most violent reaction.
Why are some reactions more
explosive?
Which ratio gives the loudest
explosion? Why?
Follow CLEAPSS risk assessment.
Wear ear plugs!
Reacting Masses
Experiment
The change in mass when magnesium burns
http://www.youtube.com/watch?v=
d8QWXCaXfSs
Analysing results
1. Record class results in a spreadsheet and produce a
line of best fit.
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Show that there is a fixed ratio of Mg:O
2. Plot individual results onto a pre-prepared graph
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Identify the most likely formula
3. Wait until mole has been covered and calculate
empirical formula
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Calculate the formula MgO
Experiment
Analysing Epsom salts
A weighed sample of Epsom salt (magnesium sulphate) is
heated in a crucible to drive off the water of crystallization.
Simple graphs could show that there is a fixed ratio of salt
to water.
Later, (using moles) the formula of the salt could be
determined.
Introducing the ‘mole’
Introducing ‘the mole’
Thinking it through …
Take two elements:
Cu
S
(Ar = 64)
(Ar = 32)
Introducing ‘the mole’
Thinking it through …
Take two elements:
What would be the ratio of the
relative masses if you had
• 1 atom ofQ.
each element?
Is it possible to
weigh one atom
in our lab?
Cu
S
(Ar = 64)
2
(Ar = 32)
:
1
Introducing ‘the mole’
Thinking it through …
Take two elements:
What would be the ratio of the
relative masses if you had
• 1 atom of each element?
• 100 atoms of each element?
• 1000 atoms of each element?
• 1 million atoms of each element?
Cu
S
(Ar = 64)
2
:
(Ar = 32)
1
Introducing ‘the mole’
Thinking it through …
Q.
What is the ratio of the
actual masses of the two
elements?
Q.
What can you say about
the numbers of atoms in
the two samples? How do
they compare?
64 g of Cu
32 g of S
Avogadro’s number
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In the samples of Cu and S:
the atoms are very small
the number of atoms present is very large
the number of atoms present is called Avogadro’s
number (6.02 x 1023)
we refer to this number and the physical amount of
substance (measured in g) as ‘one mole’
See the ‘Astounding numbers’ activity from Chemistry for
the Gifted and Talented (Tim Jolliff, RSC 2007).
Magic Triangles
Working out the relationship
Q. What is the mathematical relationship between moles, Mr and mass?
Reacting quantities
Calculate the yield of copper sulphate – worksheet 13.
Moles & Gases
Avogadro’s Law (hypothesis/theory
Equal volumes of gases at the same temperature and pressure
contain the same number of molecules.
Turning this around:
Equal numbers of molecules (measured under the same conditions)
will occupy the same volume
So,
Avogadro’s number of molecules (a mole) of any gas will occupy
the same volume.
At room temperature and pressure this volume is 24dm3.
Experiment
Can we estimate the Avogadro volume?
A quick and easy method using a known mass of zinc carbonate to
produce carbon dioxide, the volume of which is measured in a gas
syringe
Experiment
The volume of 1 mole of
hydrogen gas
A method using
magnesium and
hydrochloric acid and
simple apparatus found in
most labs.
Celebrate the Mole
National Mole Day is October 23rd.
http://www.moleday.org/
Celebrated from 6.02 am to 6.02 pm
Thus 6.02 10/23
How could you celebrate it?
Additional resources and links
http://www.ulster.ac.uk/scienceinsociety/molepack.html
Mole Day Pack
http://www.rsc.org/images/Misconceptions_update_tcm18188603.pdf
‘Beyond appearances’
http://www.rsc.org/Education/Teachers/Resources/Books/Misco
nceptions.asp
Selected activities from
RSC Misconceptions
http://www.rsc.org/Education/Teachers/Resources/Aflchem/
Assessment for Learning
Chemistry
http://www.mindsetsonline.co.uk/product_info.php?products Mindsets (for the Ionic
Jigsaws)
_id=1141
http://www.cleapss.org.uk/
CLEAPSS website link to
their homepage