Iven the balanced equation representing a reaction: mg (s) + ni2 + (aq) → mg2 + (aq) + ni (s) what is the total number of moles of electrons lost by mg (s) when 2.0 moles of electrons are gained by ni2 + (aq) ?

This is a redox reaction where oxidation and reduction take place. Here, Mg (s) oxidizes into Mg²⁺ (aq) by increasing oxidation number from 0 to + 2 while Ni²⁺ (aq) reduces into Ni (s) by decreasing oxidation number from + 2 to 0.

Oxidation

Mg (s) → Mg²⁺ (aq) + 2 e

Reduction

Ni²⁺ (aq) + 2 e → Ni (s)

Hence, 2.0 moles of electrons lost by Mg (s) when 2.0 moles of electrons are gained by Ni²⁺ (aq).