2021 P1 Q27 - Calculate Avogadro Constant from Electrolysis Data

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 27.

2021 p1 q27 question

We are required to calculate Avogadro's constant from the electrolysis of molten aluminium oxide.

Let's recap the 3 Faraday's equations that we use for calculation questions in electrolysis.

2021 p1 q27 faradays equations

Q = I.t
where Q = charge (C)
I = current (A)
t = time (s)

Q = n.F
where Q = charge (C)
n = number of moles of electrons (mol)
F = Faraday's constant (96500 C mol-1)

F = e.L
where F = Faraday's constant (C mol-1)
e = charge of 1 electron (1.60 x 10-19 C)
L = Avogadro's constant (6.02 x 1023 mol-1)

The values of F, e and L can all be found in the Data booklet.

First we need to determine the moles of electrons involved in this electrolysis.

From the mass of Al we can calculate moles of Al.

Using reduction half equation of Al3+ to Al, we can compare mole ratio between electron to Al to determine moles of electrons.

2021 p1 q27 calculate mole of electrons

Next we can use equation Q = n.F to determine Faraday's constant, since charge is given.

2021 p1 q27 calculate faraday constant

Finally we can use equation F = e.L to solve for Avogadro's constant L.

2021 p1 q27 calculate avogadro constant

The calculated value (6.05 x 1023) is quite close to the actual value of Avogadro's constant which is 6.02 x 1023.

Therefore the answer to this question is option B.

2021 p1 q27 answer

Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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