Question

What weight of Copper will be deposited by passing 2 faradays of electricity through a Cupric salt? (Atomic weight of Cu= 63.5 g/mol)
a.) 2.0 g
b.) 3.175 g
c.) 63.5 g
d.) 127.0 g

Answer 1

This question is based on faraday’s first law of electrolysis. That law explains that 1 mole electrons containing the charge equal to 1 faraday and 1 faraday charge always deposits 1 equivalent of the element and 1 equivalent of a substance can be easily converted into its moles and moles into weight.

Complete step by step answer:
Faraday’s first law of electrolysis states that
Mass of element deposited on an electrode will be directly proportional to the charge pass through the electrolyte solution.
$W\propto Q$
$W = ZQ$
$W = ZIt$
Here: proportionality constant Z is the electrochemical equivalent of the element, from the experiment its value comes as follows
$Z=\dfrac{E}{96500}$
E is the equivalent weight of the element.
$W=\dfrac{E}{96500}Q$
96500 C is the faraday constant and it is called as 1 faraday charge.
If 96500 C charge is passed through an electrolyte solution then
$W=\dfrac{E}{96500}\times 96500$
$W=E$
It means when we pass 96500 C charges or 1 faraday charge from an electrolyte solution, the deposited weight of the element will be equal to its equivalent weight.
- Let’s calculate the charge on 1 mole electrons
Charge on 1 electron = $1.6\times {{10}^{-19}}$
Charge on 1 mole electrons = $6.022\times {{10}^{23}}\times 1.6\times {{10}^{-19}}$
Charge on 1 mole electrons = $9.6352\times {{10}^{4}}$
Charge on 1 mole electrons= 96352 C
This 96352 C is approximately equal to 96500 C, that’s why we say charge on 1 mole electrons is equal to 1 faraday.
- Chemical equation for the deposition of copper is :
$C{{u}^{2+}}+2{{e}^{-}}\to Cu(s)$
- According to reaction stoichiometry 2 mole electrons deposit 1 mole of Copper metal. As we know 1 mole electron contains 1 faraday charge, so 2 mole electrons will contain two faraday charges.
Therefore two faraday charges will deposit 1 mole of copper metal.
Deposited quantity of Copper metal = 1 mole
Weight of 1 mole Copper = 63.5 g
Weight of Copper deposited = 63.5 g
So the correct answer is “C”:

Additional Information: Electrolysis is a very important process in metallurgy. This process is often being used to get ultra pure metals. For purification of different metals different processes are available. For copper we do electrolytic refining and for Aluminum we have hall-heroult process and serpek process and for some highly reactive metals like sodium we directly get them by electrolysis of their ore like rock salt.

Note: To solve these type of questions always write balanced equation of metal deposition and check on the reactant side how many moles of electrons are present and according to that number calculate the number of moles of metal deposited based on the how many faraday charge is passing through the solution. That is the short trick to solve such questions. Like over here if in place of 2 faraday, 4 faraday charges would have been passing through the solution then deposited metal quantity were 2 moles.