Question

In an experiment like the one you will complete, a student collected the following data. Use this information to complete the questions below. YOU MUST SHOW ALL CALCULATIONS.

Change in Mass of Anode: -0.023 g

Change in Mass of Cathode: +0.019 g

Average Current: 38.7 mA

Time of Electrolysis: 1860 s

d) Using the average of the change in mass of copper from the electrodes in the electrolytic cell, calculate the molar mass of copper.

Answer 1

At anode , oxidation occurs, i.e.

Cu(s) --------> Cu²⁺(aq) + e^-

Thus, equivalent mass of Cu = molar mass of Cu/2

Number of equivalents of Cu in 0.023 g of it = 0.023/equivelnt mass of Cu = 0.046/molar mass of Cu

Now, as per Faraday's Law,

1 equivalent of Cu requires 96500 C of charge and vice versa

Now, total charge passed = current *time in sec = 0.0387*1860 = 71.982 C

Thus, total equivalents of Cu dissolved = 71.982/96500 = 7.46*10^-4

Thus, (0.046/molar mass of Cu) = 7.46*10^-4

or, molar mass of Cu = 61.67 g/mole................(1)

Now,

At cathode, reduction reaction occurs i.e gain of electrons

Cu²⁺(aq) + 2e^-   ----------------> Cu(s)

Thus, equivalent mass of Cu = molar mass of Cu/2

Number of equivalents of Cu in 0.019 g of it = 0.019/equivelnt mass of Cu = 0.038/molar mass of Cu

Now, as per Faraday's Law,

1 equivalent of Cu requires 96500 C of charge and vice versa

Now, total charge passed = current *time in sec = 0.0387*1860 = 71.982 C

Thus, total equivalents of Cu dissolved = 71.982/96500 = 7.46*10^-4

Thus, (0.038/molar mass of Cu) = 7.46*10^-4

or, molar mass of Cu = 50.94 g/mole................(2)

Thus, average molar mass of Cu = {(1) + (2)}/2 = 56.3 g/mole