1.57 calculate the amounts of the products of the electrolysis of molten salts and aqueous solutions.

Electrolysis Calculations

Using moles = mass / RFM

The charge on 1 mol of electrons is 96500C.
96,500C/mol is called Faradays Constant.

When copper is deposited during electrolysis:

Cu²⁺ + 2e^-  à Cu

So for every mol of copper 2 mols of electrons are needed to discharge it. 

(The number before the symbol represents the number of moles e.g 2e^-)

96,500C x 2 is the quantity of charge required for discharge. 

To find the amounts of products of electrolysis:

Use moles = mass/RFM

e.g.

Find the mass of metal deposited when 1F of electrons flows through Silver Nitrate:

Ag⁺ + e^- à Ag

So 1 mol of electrons is required to discharge 1 mol of Ag ions

mass = moles x RFM

mass = 1 x 108 = 108g

Find the amount of metal deposited when 1F of electrons flows through 5g of Nickel (II) Nitrate:

Ni²⁺ +2e^- à Ni

So 1 mols of electrons are required to discharge 0.5 mole of Ni ions 

(always use 1 mol of electrons and adjust the other figures accordingly)

mass = moles x RFM

mass = 0.5 x 59 = 29.5g of Ni

Using C = I x t

e.g.

A current of 0.01 amperes passes for 4 hours through a solution of gold (III) nitrate. What mass of metal is deposited.

Total Quantity of Charge = 0.010 x (4 x 60 x 60)

C = 144C

 

Equation: Au³⁺ + 3e^- à Au

So 3 moles of electrons are required to discharge 3 moles of gold

3 x 96,500C deposits 1 mol (197g) of gold

144C deposits 197(mass) / 3 x 96,500(quantity of charge required to discharge) x 144g of gold  (because of the coulombs)

144C = (197/3x96500) x 144g = 0.098g

NB - a good way of tackling these questions is to look at the question, see what it's asking you, see what its given you and to see which formulas you should use e.g. moles = mass / RFM