Please check what i\'m doing wrong on this problem: Chromium metal (M.W = 52.00
ID: 683545 • Letter: P
Question
Please check what i'm doing wrong on this problem:Chromium metal (M.W = 52.00 g/mole) is electroplated from acidicaqueous solutions containing the dichromate ion,Cr2O72-. What is the minimum time needed toplate out 10.0 g of chromium metal from such a solution, if thecurrent is 50.0 A?
my half-reaction : Cr2 + 2e- --> Cr (s) forproblems like this , how do I know if it's supposed to be writtenas oxidation or reduction? (i guessed here that it'sreduction)
finding the charge:
F = Faraday's constant = 9.65 x 10^4 C/mole e-
10.0g Cr x (1 mol Cr/ 52.0 g/mol) x (2 mol e-/ 1 mol Cr) x (9.65 x104C / 1 mole e-) = 3.7 x 10^4 C
finding time:
current x time = charge so time = charge / current => time= 3.7 x 10^4 C / 50.0 A = 742 s
the answers given:
a. 372 s
b. 744 s
c. 1116 s
d. 1482 s
e. 2227 s thisis the correct one!
please show me how to solve this problem, step by step, b/c I'mgetting the wrong answer. Thank you :)
Explanation / Answer
Your mistake is in your half-reaction. Cr2O7 2- doesnot have Chromium in a +2 oxidation state. I'm not sure whatclass this is for, but in Inorganic Chemistry you learn that firstyou find the charge of the non-transition metals, and then use thetransition metal to balance the total charge. You want a total of -2 obviously. Oxygen is always -2. Multiply -2 by 7 and get -14. Addtwo because the end answer is -2. This gives 12. Divide 12 by 2, because there are two Chromium atoms. Thisgives Chromium 6+. Multiply your answer by 3 and you'll get the number you're tryingto get. Everything else you did looks good.