Homework Help: Gen Chem

Posted by Sarah H on Friday, March 28, 2008 at 5:39pm.

A current is passed through two cells connected in series. The half-reactions occurring at the cathode of each cell are

TcO4-(aq) + 8 H+(aq) + 7 e- ===> Tc(s) + 4 H2O(l) and
Cu2+(aq) + 2 e- ===> Cu(s).

If 4.40 g of copper are deposited in the second cell, how much technetium, Tc, is formed in the first cell?

I am not sure what equations or where to begin to solve this.

Gen Chem - DrBob222, Friday, March 28, 2008 at 5:46pm
Use the information from the deposition of Cu to determine the equivalents of Cu deposited, then the number of coulombs. The colulombs will be the same for the Tc half cell and you can use that to determine the mass of Tc deposited.
Gen Chem - Sarah H, Friday, March 28, 2008 at 8:52pm
How can I figure the equivalents of Cu deposited and then the number of coulombs?
Gen Chem - DrBob222, Friday, March 28, 2008 at 10:04pm
You need to recalculate since I have estimated along the way.
1 equivalent of Cu = 63.546/2 = about 32.
96,485 C needed to deposit 32 g Cu.
How many C needed to deposit 4.40 g?
That must be 96,485 C x (4.40/32) = about 13,000 C.
That is the same # C that will ppt Tc or about 13,000 C will ppt Tc.
1 equivalent of Tc = 98/7 = about 14
So 96,485 C will ppt 14 g Tc, how much will 13,000 C ppt?
Check my thinking. Check my arithmetic. Redo all the numbers since I've estimated.

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