Calculation of Molar Mass

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1. If you measure the current (I) indirectly by observing the voltage (V) drop across a known resistor ®, convert the voltage readings to current using Ohm’s Law. The electric current (in amps) passing through the resistor

   (and through the cell) is directly proportional to the voltage drop (in volts) across the resistor:

    

   I = V/R

    

   The proportionality constant (l/R) is where R is the resistance expressed in ohms.

2. Graph the electric current (in amps) on the y-axis against time (in seconds) on the x-axis. The total charge that passed through the electrolysis cell is given by the area beneath this curve. If the current is constant, this area is:

    

   Q = area = I x t

   Calculate this charge in coulombs.

3. Convert the coulombs of charge to mol electrons:

    

   N =

     

4. The equation for the reduction half-reaction responsible for the plating at the cathode is

    

   Cu²⁺ (aq) + 2e^- ® Cu(s)

    

5. Use the mol ratios of the preceding balanced equation to calculate the number of moles of Cu plated out:

   n(Cu) = n/2

    

6. Use the initial and final weighings of the copper mesh electrode to calculate the mass of copper plated out:

    

   m(Cu) = m(final) – m(initial)

    

7. Calculate the molar mass (M) of copper:

1. M =