E Determining Avogadro's Number

E
Mole Day Planning Pack
Determining Avogadro’s
Number using Electrolysis
This experimental set up provides you with all the information you need to determine
Avogadro’s number. If you need any help, use the hint cards. These are structured to help you
understand how you get from your measurements to the answer. Remember to wear safety
glasses throughout the experiment.
Clean and dry a pair of copper electrodes.
wires
copper
sulphate
solution
Find the dry mass of the anode before you start the experiment.
Set up the apparatus below. Use 0.1 M CuSO4 (aq) (harmful) as the electrolyte. You will also
need a stopwatch.
Switch the power supply on, starting the stopwatch at the same time. Record the current
(about 0.4 A, but record the exact current you read off the ammeter) and leave for about 35
minutes.
Find the mass of the anode when dry at the end of the experiment.
While you are waiting, write down the steps you are going to take to calculate Avogadro’s
number.
Calculate Avogadro’s number using your experimental data. Compare your value with the
value 6.02 x 10 23 . Identify sources of error.
Information
Remember to complete a risk assessment
before beginning practical work.
A
power supply
beaker
carbon electrodes
e charge on 1 electron / C
n chemical amount / mole
Oxidation of copper at anode:
Cu (s) Cu ÷
÷
÷
2+ (aq) + 2e- N number of atoms NA = nCu NCu
NA Avogadro’s number Molar mass of Copper = 63.55 g mol-1 Q charge / C e = 1.60 x 10-19 I current / A
Q = It
C
Number of electrons = Q e
n = mass/g molar mass/ g mol-1 n = No. elementary entities NA
Extension activity: The faraday, F, is defined as the amount of electrical charge on one mole of
electrons. What value does your experiment give you for the Faraday? Compare it with the
literature value 96500 C mol-1 ÷
(3 SF).
www.ulster.ac.uk/scienceinsociety/ 13
ANALYSING PROBLEM SOLVING
S

Mole Day Planning Pack
Determining Avogadro’s Number using Electrolysis
Hint cards
Cut one set of cards out for each table. Fold along the dotted line and place the cards face
down (number facing up) on the table. Students can refer to the hint cards if they have
problems in deciding their strategy for answering the question.
1
Avogadro’s number
2
Amount of copper
(in moles)
3
Number of atoms of
copper
4
Number of electrons
(a)
5
Number of electrons
(b)
Avogadro’s number
You can calculate Avogadro’s number from the equation:
÷
NA = nCu NCu
You will need to find the number of moles of copper (see card 2) that
take part in this reaction and the number of atoms of copper (see card
3) that take part in this reaction.
Amount of copper (in moles)
To find the number of moles of copper that took part in electrolysis
you can use the equation
÷
n = mass/g molar mass/ g mol -1
The mass of copper will come from your experiment.
Mass of copper that took part in electrolysis = Mass of anode at start
– mass of anode at end
Number of atoms of copper
Look at the equation for oxidation at the anode in the information
box. This links the number of atoms of copper with the number of
electrons that take part in electrolysis.
What is the ratio of electrons to copper atoms?
If you are stuck on how to find out the number of electrons that take
part in electrolysis, see card 4.
Number of electrons (a)
Electric current consists of electrons carrying charge through a wire.
To calculate the total charge that is passed through the wire, you will
need to use the equation linking current, charge and time.
Substitute your values from your experiment. To find out how to
convert from charge to number of electrons see card 5.
Number of electrons (b)
The information table tells you the charge of one electron. You have
just calculated the total charge.
Now calculate how many electrons took part in electrolysis using the
relationship:
Total charge = Number of electrons x Charge of one electron
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ANALYSING PROBLEM SOLVING
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