Chromium plating is applied by electrolysis to objects suspended in a dichlromate solution , according to following `(` unbalanced `)` hald reaction `:`
`Cr_(2)O_(7)^(2-)(aq) +e^(-) +H^(o+)(aq) rarr Cr(s)+H_(2)O(l)`
How many hours would it take to apply a chromium plating of thickness `2.0xx10^(-2)mm` to a car bumper of suface area `0.25m^(2)` in an electrolysis cell carrying a current of `75.0A?`
`[` Density of chromium is `7.19g cm^(-3)]`

Options

A. `2.2h`
B. `1.5h`
C. `3.0h`
D. `0.25h`

Answer

b

Explanation

Volume to be covered
`=(0.25xx10^(4)cm^(2))xx(2xx10^(-3)cm)=5cm^(3)`
`implies` Mass of `Cr` to be deposited `=5xx7.19g`
`=35.95g`
`-=(35.95)/(52)mol`
`=0.69mol`
`Cr_(2)O_(7)^(2-)+14H^(o+) +12e^(-) rarr 2Cr+7H_(2)O`
Now, `2 mol Cr` deposited `-=12 F` electricity
`implies 0.59 mol Cr` deposited `-=(12)/(2)xx0.069F`
`=(It)/(96500)=(75xxt)/(96500)t=1.48h~~1.5h`