What is the voltage of the electrochemical cell written as: cu(s) | cu2+(aq) || mg2+(aq) | mg(s)?
a.2.71 v
b.-2.71 v
c.-2.03 v
d.2.03 v

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The best and most correct answer among the choices provided by the question is  B.-2.71 V.  
Mg2+(aq) + 2e- -> Mg(s) E=-2.37 V  

Cu2+(aq) + 2e- -> Cu(s) E =+ 0.34 V  

Since Cu is acting as the anode, the equation needs to be reversed.  

Cu(s) -> Cu2+(aq) + 2e- E =- 0.34 V  

Ecell= -2.37 V+ (- 0.34 V) = -2.71 V  

Hope my answer would be a great help for you.