Determining the Avogadro constant
Requirements per student (or pair)
Measuring cylinder, 10cm 3
Teat pipette drawn out to a fine point or teat
pipette with adapter
Adapters called ‘microtips’ can be obtained
from Hughes and Hughes Ltd
The teat pipette should be capable of delivering
about 100 drops to 1cm 3
Large trough – glass or earthenware
Safety glasses
Fine cotton thread, 40-50cm
Scissors
Vaseline or petroleum jelly
Oleic acid solution in pentane (0.005%) (in
corked test tube)
About 5cm3. 0.50cm3 of oleic acid made up to
50.0cm3 with pentane and then 0.50cm 3 of this
solution made up to 100.0cm 3 with pentane for
the final solution
If pentane is not available, petroleum ether (4060 or even 30-40) can be used
Hazard Warning!!
Pentane is extremely flammable and harmful by inhalation. Therefore you must:
 Keep the stopper on the bottle when not in use
 Keep the liquid away from flames
Candidate Name:
Date:
IB Practical - Determining the Avogadro constant
Aim
The purpose of this experiment is to estimate the value of the Avogadro constant
and to compare this estimate with the accepted value.
Introduction
When a solution of oleic acid (more correctly called cis-octadec-9-enoic acid),
C17H33CO2H, in pentane is dropped onto water, the pentane evaporates leaving
behind a layer of oleic acid one molecule thick. For this reason, this experiment
has been called ‘The Molecular Layer Experiment’.
You use a loop of hair or thread to contain the oleic acid and to give a measure
of the surface area. By making certain assumptions about the shape of the
molecule and its alignment on the surface, you can get a reasonably accurate
value for the Avogadro constant.
The experiment has two parts. In the first, you calibrate the pipette. This gives
the volume of one drop of solution. In the second part you determine how many
drops of solution are required to just fill the loop with a layer of oleic acid
molecules. Then you are guided, step by step, through the calculation.
Assessment
You are not being formally assessed on this practical activity. However, the time
you spend on this practical will count towards the necessary hours in IB
Chemistry.
Requirements








Safety glasses
Measuring cylinder, 10cm3
Teat pipette (maybe with adapter)
Trough
Long human hair or cotton thread
Scissors
Vaseline or petroleum jelly
Oleic acid solution in pentane (0.05cm 3 of oleic acid per dm 3)
Hazard Warning! – Pentane is extremely flammable and harmful by inhalation. Therefore you
must:
 Keep the stopper on the test tube when not in use;
 Keep the liquid away from flames
Procedure
1. Fill the teat pipette with oleic acid solution and deliver it drop by drop into the
10cm3 measuring cylinder. Count the number of drops which must be delivered
from the pipette to reach the 1cm3 mark. Enter your value in the results table.
2. Tie the hair or cotton thread in a loop. Use a reef knot rather than an overhand
knot so that the loop will make a flat circle.
Cut the ends as close to the knot as
possible. Hair is preferred because it does
not need greasing but if you are using
thread, thoroughly but lightly grease it with
Vaseline. It is most important that no part
of the thread escapes greasing. Run the
knotted thread through your fingers
several times before wiping off the excess
Vaseline.
3. Fill the trough with water and float the loop on it, making sure that the entire
circumference is in contact with the surface. Look very carefully for ‘bridges’ or
submerged loops and move them into the surface with a clean glass rod or a
pencil point.
4. Using the same pipette, add the oleic acid solution drop wise to the middle of the
loop until it is filled. At first you will probably see the loop expand to a circle and
then retract again. Before the loop is filled, it ‘gives’ when you push it gently from
the outside with a pencil. When the loop is filled it will slide across the surface,
only denting very slightly when pushed gently with a pencil. Count the number of
drops required to fill the loop and record this in the results table.
5. Measure the diameter of the loop and complete the results table.
6. If you have time, repeat the whole procedure. However, you must use a fresh
hair or thread and wash out the trough thoroughly to obtain a clean surface.
Results table
Number of drops to deliver
1cm3 of solution
Number of drops delivered to
make a monomolecular layer
Diameter of monomolecular
layer/cm
Calculation
1. Calculate the volume of 1 drop delivered from the teat pipette using the value in column 1
of the results table.
Volume of 1 drop =
cm3
2. Calculate the volume of oleic acid in 1 drop of solution delivered from the teat pipette.
Remember that 1000cm 3 of this solution contains 0.05cm 3 of oleic acid.
Volume of oleic acid in 1 drop =
cm3
3. Calculate the volume of oleic acid delivered to make the monomolecular layer; i.e. the
volume of oleic acid in 1 drop x the number of drops required.
cm3
Volume of oleic acid in monolayer =
4. Calculate the surface area of the oleic acid layer.
Area = πd2/4 =
cm2
5. You know the volume of oleic acid (from step 3) and the surface area it covers (from step
4). It is a simple matter to calculate the thickness of the layer because volume = area x
thickness.
Thickness =
cm
6. Calculate the volume of one molecule of oleic acid by assuming that it is a cube with
sides equal to the thickness of the layer.
Volume of one molecule =
cm3
7. Calculate the volume of one mole of oleic acid given that its density if 0.890 g cm -3 and its
molar mass is 282 g mol-1.
Volume per mole of oleic acid =
cm3 mol-1
8. Divide the volume per mole by the volume of one molecule to determine the Avogadro
constant.
Avogadro constant, L =
mol-1
Questions
1. Suggest some sources of error in this experiment which account for this discrepancy
between the value of L that you obtained and the accepted value of L = 6.02 x 10 23 mol-1.
2. Which of the values that you used in your calculations is subject to the greatest error?
3. Pentane is not the only liquid that can be used in this experiment. Suggest four properties
which a suitable substitute must have.
Teacher’s Notes
Hair seems to give better results than cotton and does not need initial greasing. It is important to
make sure that the loop is in contact with the surface throughout its circumference – otherwise
the oleic acid will leak out.
The students should obtain a value for L to within one power of 10 of the accepted value. You
might ask more able students to investigate quantitatively the effect on the calculation of making
various assumptions about the shape of the molecule.
Specimen Results
Number of drops to deliver
1cm3 of solution
Number of drops delivered to
make monomolecular layer
Diameter of monomolecular
layer / cm
100
12
11.2
These results will provide an answer of 1.4 x 1024 mol-1
Answers to Questions
1. The various sources of error which may account for the difference between the
experimental and actual value for L are as follows:
a) the number of drops required to make a monomolecular layer was inaccurately
measured
b) not all the pentane had evaporated from the surface
c) the volume of the drop was inaccurately determined
d) the molecules were unevenly dispersed on the surface of the film. I.e. there was
more than one layer of molecules on parts of the film
e) the assumption about the shape of the molecule was incorrect. In fact, the
molecule is rather more the shape of a cylinder than a cube
2. The number of drops required to fill the loop is subject to the greatest error. Even if
students are sure to the nearest drop, this gives only two significant figures (or only one
for a small loop!) whereas all the other values are obtained to three significant figures.
3. A substitute for pentane must:
a) dissolve oleic acid readily
b) evaporate readily
c) not react with oleic acid
d) not react with water
e) not dissolve in water