Answer: D) NaCl
Explanation: In given answer choices the metals are Au, Ba, Cu and Na. Cl has -1 charge and sulfate ion has -2 charge.
Since there are three Cl atoms for each Au, the charge on Au is +3. Similarly, the charge on Ba is +2, charge on Na is +1. There is one sulfate ion for each Cu and so the charge on Cu is +2.
In electrolysis, the metals are deposited when they are reduced on accepting electrons. The half equations for all the above four metals are written as:
\(Au^+^3(aq)+3e^-\rightarrow Au(s)\)
\(Ba^+^2(aq)+2e^-\rightarrow Ba(s)\)
\(Cu^+^2(aq)+2e^-\rightarrow Cu(s)\)
\(Na^+(aq)+1e^-\rightarrow Na(s)\)
Looking at all the four equations, it's only the last equation that involves one electron. So, one faraday of electricity will liberate one gram-atom of sodium metal from a solution of NaCl.
