24.1.2 Faraday's Law and Avogadro

6. Determining Avogadro’s Constant () Experimentally

Apparatus:

• Pure copper anode and cathode.
• Aqueous Copper(II) Sulfate electrolyte.
• DC power supply, ammeter, variable resistor (to keep current constant).

Determining Avogadro

Method:

1. Weigh the pure copper anode before the experiment.
2. Pass a known constant current () for a known time ( in seconds).
3. Remove the anode, wash with distilled water, dry with propanone (evaporates quickly), and reweigh.
4. Calculate mass lost by anode calculate moles of Cu dissolved ().

Calculation:

• Half equation: (1 mol Cu releases 2 mol ).
• Moles of electrons passed = .
• Total charge .
• Charge of 1 mole of electrons (Faraday, ) =
• Avogadro’s Constant

Why use the Anode?

Past papers frequently ask why we measure the mass loss of the anode rather than the mass gain of the cathode. Answer: Copper deposited at the cathode does not always stick properly and can flake off, leading to inaccurate (lower) mass readings. The mass lost from the anode is a much more reliable metric.