Acr3+(aq) solution is electrolyzed using a current of 7.40 a. what mass of cr(s) is plated out after 1.85 days?

When Cr+3  + 3e-  → Cr(s)

So according to the charge formula:

charge Q = I * t 

when we have I (the current) = 7.4 a

and t (the time in seconds) = 1.85d * 24h * 60 min*60sec=  159840 sec

by substitution:

Q = 7.4 a * 159840 sec = 1182816 C

now to get the mass we have to use Faraday constant which is:

F = e NA

   = 1.602 x 10^-19 * 6.022 x 10^23

  = 96485 C mol^-1

when e is the electron charge & NA Avogadro's number for the charge in on mol 

and as we can see from the equation Cu3+ reacts with 3 electrons so,

the mass of Cr(s) = Q* (1mole-/F) * (1molCr / 3 mol e-)

                            = 1182816 * (1/96485) * (1/3)

                            = 4.1 g