Question

45. Cadmium amalgam is prepared by electrolysis of a solution of CdCl2 using a mercury cathode.
How long should a current of 4 A be passed in order to prepare 10% by wt. Cd in Cd-Hg
amalgam on cathode of 4.5 g Hg? (atomic wt. of Cd = 112)
(a) 400 sec (b) 215.40 sec (c) 861.6 sec (d) 430.8 sec​

Answer 1

Answer : The correct option is, (b) 215.40 sec

Explanation :

First we have to calculate the mass of Cd.

10 % by wt. Cd in Cd-Hg means that 10 gram is Cd and 90 gram Cd-Hg.

4.5 g Hg require Cd to prepare 10 % amalgam = $\frac{10}{90}\times 4.5=0.5g$

The half-reaction of cathode will be :

$Cd^{2+}+2e^-\rightarrow Cd$  

As, 1 mole of electrons gives 96500 C charge

So, 2 mole of electrons gives $2\times 96500C$ charge

Now we have to calculate the charge required.

Charge required to deposit 0.5 g of Cd  = $\frac{2\times 96500}{112}\times 0.5$ coulomb

Now we have to calculate the time.

$Charge=Current\times Time$

$Time=\frac{2\times 96500\times 0.5}{112\times 4}=215.40s$

Therefore, the time required will be, 215.40 seconds.