Question about a prob

Question

How long will it take to deposit 0.00235 moles of gold by the electrolysis of KAuCl4

using a current of 0.214 amperes

well i know how to solve the problem already but

when theyre given us KAuCl4, how do we know if we should use this half rxn: Au^3 3e- -> Au and not Au e- -> Au

Answer 1

You have to consider the silver you are starting with.  The equation KAuCl4 tells you that you have a charge of -4 from the chlorine ions and a charge of +1 from the potassium ion.  That leaves you with a +3 charge difference.  Since you are working with a natural compound that entire +3 charge must be coming from the silver.

Therefore, you must be dealing with Au3+ and you have to use the appropriate half reaction.