Question

Metallic magnesium can be made by the electrolysis of molten MgCl2.

a. What mass of Mg is formed by passing a current of 5.35 A through molten MgCl2 for 3.30 days?

b. How many minutes are needed to plate out 18.00 g Mg from molten MgCl2, using 3.20 A of current?

Answer 1

a)

Mg2+ + 2e- ------> Mg

1 mol of Mg requires 2 mol of electron

1 mol of electron = 96485 C

So,1 mol of Mg requires 192970 C

let us calculate the charge passed:

t = 3.3 days = 3.3*24*3600 s = 285119 s

time, t = 285119s

Q = I*t

= 5.35A * 285119s

= 1525386.65 C

mol of Mg plated = 1525386.65/192970 = 7.90479 mol

Molar mass of Mg = 24.31 g/mol

mass of Mg = number of mol * molar mass

= 7.90479 * 24.31

= 192 g

Answer = 192 g

b)

Mg2+ + 2e- ------> Mg

1 mol of Mg requires 2 mol of electron

1 mol of electron = 96485 C

So,1 mol of Mg requires 192970 C

let us calculate mol of element deposited:

use:

number of mol, n = mass/molar mass

= 18/24.31

= 0.74044 mol

Now calculate total charge passed

total charge = mol of element deposited * charge required for 1 mol

= 0.74044*192970

= 142881.94 C

use:

time = Q/i

= 142881.94/3.2

= 44651 seconds

= 44651/60 minutes

= 744 minutes

Answer: 744 minutes