[Rutherford]

How much time is needed when chromic acid is electrolysed (density of the current is 50A/dm², 40% of the current is used), so that an area of 150cm² is coated with a layer of Cr (thickness is 0.2mm)? Density of Cr is 7.1gcm^-3.

The volume is V=150*0.02=3cm³, so the mass of Cr is m=7.1*3=21.3g, n=21.3/52=0.41mol
nFz=It therefore t=nFz/I
n is calculated, F=96500C/mol, z=6, I is calculated: I=50A/dm²*0.4*0.15dm²=3A
When I put all this I get that t≈22h, but the answer is 2.16h, much lower than I got. Where did I mistake?

[Borek]

How many cm² in dm²?

[Rutherford]

Yes , it is 1.5dm². Then t=2.2h, so it is okay. Thanks for the correction.