118.8cm² surface of the copper cathode of a voltameter is to be coated with 10^-6m thick copper of density 9 x 103kgm−³. How long will the process run with 10A constant current? [3.3 x 10^-7kgC‾¹]

A. 5.4 min
B. 10.8 min
C. 15.0 min
D. 20.0 min

Correct Answer:

Option A – 5.4 min

Explanation

From faradays law, M = itz
But mass, M = volume x density
and volume = cross sectional area x thickness
M = 118.8cm² x 10^-6 x 9 x 10³
∴ 118.8cm² x 10^-6 x 9 x 10³ = 10 x t x 3.3 x 10^-7
∴ (118.8 x 9) x 10^-7 = 10 x t x 3.3 x 10^-7

∴ t = 118.8×93.3

t = 324 seconds

Convert to minutes by dividing by 60

t = 32460

t = 5.4 sec