electrochemistry/faraday's laws help

[lorelai]

There is a style of question that recurs in many of the multiple choice questions that looks something like this:



It's three cells connected to an external power sources in a series. The first contains Mg(NO₃)₂, the second contains Ni(NO₃)₂, and the third containsAgNO₃ - all at a concentration of 1.0 M and using platinum electrodes.
5.79 x 10⁵ Coulombs of electricity passes through each of the cells. The ratio of masses of the metals produced would be?

I'd appreciate some general help on how to approach this question (:

[tony3272]

First thing you want to do is to see which ones will actually result in the reduction of metal ions. i.e by looking at your electrochemical series, Mg²⁺ is the only on out of the three which is a weaker oxidant than water. This means that no metal will be produced at this electrode as H₂O is reduced in preference.

So that rules out C and D straight away.

To work out the amount in mol of electrons, simply use the charge that is given.
n(e^-)==6mol

So the two reduction reactions that will occur are:
Ag⁺+e^- ---> Ag_(s)
Ni²⁺+2e^- ---> Ni_(s)

This means that 6 mol of Ag is produced and 3 mol of Ni is produced by looking at the ratio.
So next step is to work out the mass of each one and compare, which you'll see that the ratio is 0 : 1 : 4

i.e A

[lorelai]

Thank you! I knew I needed to use that rule, but before I didn't really know what it was telling me (: