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Question:

A current of 10.0 A flows for 2.00 hrs through an electrolytic cell containing a molten salt of metal X. This results in the decomposition of 0.250 mol of metal X at the cathode. The oxidation state of X in the molten salt is: (F=96500C)

1

2

4

3

Solution:

t=2×60×60=7200s
Charge=current × time=10 A × 7200 s=72000 C
According to the reaction:
Xn+(aq) + ne− → X(s)
We require n × 96500 C to deposit 1 mol of X.
For 0.25 mole of deposition we require n × 24125 C.
Hence, n = 72000/24125 = 3