A galvanic cell constructed from a Mg electrode in 1.0 M Mg(NO3)2 and a Ag electrode in 1.0 M AgNO3 at 25°C has which standard cell potential?

• A. -3.17 V
• B. +3.16 V
• C. +3.97 V
• D. -1.57 V

Answer: (B)

In a galvanic cell, the standard cell potential is found from the difference between the standard reduction potentials of the cathode and the anode: E°cell = E°red(cathode) − E°red(anode). The electrode with the higher reduction potential acts as the cathode (gets reduced), while the other serves as the anode (gets oxidized). Here, Ag+/Ag has a positive reduction potential (+0.7996 V) and Mg2+/Mg has a strongly negative one (−2.372 V). Therefore, Ag+ is reduced at the cathode and Mg is oxidized at the anode, giving the overall cell reaction Mg(s) + 2 Ag+(aq) → Mg2+(aq) + 2 Ag(s). Compute E°cell: 0.7996 − (−2.372) ≈ +3.1716 V, which rounds to about +3.16 to +3.17 V under standard conditions. The positive value indicates a spontaneous, favorable reaction in this arrangement, matching the given option.