Mole Day Planning Pack
Starter
Ask students to think of as many words as they can beginning with the prefix
mono. What does mono mean? What does monomolecular mean?
Stimulus
Remember to complete a risk assessment
before beginning practical work.
Demonstrate the formation of a monomolecular layer of oleic acid on water in
a bowl. Using a pipette, place one drop of oleic acid from a solution of 0.05 ml
oleic acid in 1 dm3 pentane. Explain what you are doing.
Tell students that they are going to use this demonstration and some extra information
to answer some questions. Split the class into groups, with students numbered 1, 2 and
3 in each group.
Question 1
What is the length of an oleic acid molecule?
Give students time to think about the question. Then ask them to write down on an
index card one question that they will need to know the answer to in order to answer
question 1.
In groups of 3, ask students to share their questions by passing their index cards around
the group, explaining to each other why they wrote this question down.
Now give each group the information card and ask them to come up with a strategy to
answer question 1. Hint cards are available if they are unable to move on from one
step to the next. Recommend they attempt a solution before asking for hints.
Ask the number 2 from each group to explain their idea to the class. Take each idea in
turn and encourage students to decide whether it is a feasible strategy.
Allow access to the equipment for students to answer the question.
Question 2
What value does this experiment give for Avogadro’s number?
Repeat the process used to solve question 1, this time ask the number 1 from each
group to feedback the group’s strategy.
Question 3
Compare your answer for 2 with the stated value of 6.02 x 1023. What are the
uncertainties and sources of error in your calculations?
Ask them to discuss question 3 in their group and list all of their answers. After 5
minutes ask all number 3s to feedback to the class.
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ANALYSING PROBLEM SOLVING
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An enquiry approach to
determining Avogadro’s Number
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Mole Day Planning Pack
Information Card
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This part of the molecule is
hydrophilic. It can form
hydrogen bonds so is more
soluble in water than in oil.
Oleic acid is less dense than water.
A very small amount of oleic acid dropped onto the surface of water will spread
out to form a monomolecular film (a layer 1 molecule thick on the surface of the
water).
Pentane evaporates from the surface of the water.
The molar mass of oleic acid is 282 g mol-1
The density of oleic acid is 0.895 g mol-1
Area of a circle = πr2
Concentration of oleic acid = 0.05 cm3/ dm3
m= ρV
Resources that you have access to in order to answer the question.
Balance
Small measuring cylinder
Any material used in the demonstration
Question 1 hints
Question 2 hints
1
Volume = Area x length
1
Number of molecules = Number of
moles x Avogadro’s number
2
Find the volume of oleic acid in
one drop
2
Calculate the number of moles of
oleic acid.
3
Volume of 1 drop = 5 cm3 divided
by number of drops in 5 cm3
3
Number of moles = mass (g)
divided by molar mass (g mol-1)
4
Find out how many drops are needed
to reach 5 cm3
4
Calculate the mass of oleic acid
used. m = ρV
5
Measure the area of the slick
5
Calculate the number of oleic acid
molecules
6
Assume the layer of oleic acid is
one molecule thick.
6
Assume oleic acid is a cube. V =
l3.
7
Assume oleic acid is cubic
7
No. molecules = volume of oleic acid
divided by volume of 1 molecule
www.ulster.ac.uk/scienceinsociety/ 16
ANALYSING PROBLEM SOLVING
Oleic acid (see below) has a hydrophobic end (the hydrocarbon end) and
a hydrophilic end (the carboxylic acid end).
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