For which of the following aqueous salts will electrolysis produce hydrogen gas at the cathode and oxygen gas at the anode?

• A. AgNO3
• B. Na2SO4
• C. NaCl
• D. CuSO4

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the electrolysis process in aqueous solutions. During electrolysis, water and the ions from the dissolved salt can be oxidized or reduced at the electrodes. The products depend on the relative ease of oxidation and reduction of the ions present and water.
2. Step 2: Identify the possible species at the cathode (reduction) and anode (oxidation). At the cathode, either metal cations or water can be reduced to produce hydrogen gas. At the anode, either anions or water can be oxidized to produce oxygen gas.
3. Step 3: Analyze each salt: For AgNO3, Ag+ ions are reduced more easily than water, so silver metal forms at the cathode instead of hydrogen gas. For NaCl, Cl- ions are oxidized more easily than water, producing chlorine gas at the anode instead of oxygen gas. For CuSO4, Cu2+ ions are reduced to copper metal at the cathode, and SO4^2- ions are not oxidized; water is oxidized to oxygen gas at the anode.
4. Step 4: For Na2SO4, neither Na+ nor SO4^2- ions are easily reduced or oxidized compared to water. Therefore, water is reduced at the cathode to produce hydrogen gas, and water is oxidized at the anode to produce oxygen gas.
5. Step 5: Conclude that Na2SO4 is the salt for which electrolysis produces hydrogen gas at the cathode and oxygen gas at the anode, because the ions are inert under these conditions and water undergoes electrolysis.