TBR Chem #88, psg 14 - Electroplating

[Gauss44]

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88.
Approximately what mass would be expected for a solution of Cd2+, if it were exposed for one minute to the same current and electrode used in the experiment?

C. A mass greater than 5.360 grams, but less than 6.182 grams
D. A mass greater than 6.182 grams

Highlight for correct answer: C

From passage,
A student sets up an experiment to determine the effects of the cell potential, concentration, and current on the degree of electroplating. In the first experiment, the student sets up six solutions with the following contents:

I: .1 M AgNO3 aq
II: 1.00 M AgNO3 aq
III: .1 M ZnSO4 aq
IV: 1.00 ZnSO4 aq
V: .1 M CuSO4 aq
VI: 1.00 M CuSO4 aq

Into each solution, an inert electrode weighing exactly 5grams is inserted. A steady current of electricity is applied to each electrode for the duration of one minute for all six solutions. The electrode is then removed and the mass is measured. The degree of plating can be determined by subtracting the original mass of the electrode from the final mass of the plated. electrode. The mass for each electrode after it is removed is listed below.

I: 6.187 grams
II: 6.182 grams
III: 5.360 grams
IV: 5.358 grams
V: 5.347 grams
VI: 5.350 grams

Answer Key: To form the same mass as the silver cation, cadmium would have to have twice the atomic mass of silver. (Why?) The atomic mass of cadmium (112g/m) is less than twice the atomic mass of silver (2 x 107 g/m), so the mass of Cd plated electrode muste be less than 6.182 grams (the mass of the lightest silver plated electrode).

 

[sazerac]

Answer Key: To form the same mass as the silver cation, cadmium would have to have twice the atomic mass of silver. (Why?)

Because silver has a charge of +1 and Cd has a charge of +2.

You are moving a fixed number of electrons, therefore you will get half as many Cd atoms as you will silver atoms. Sure if Cd weighed twice as much as silver you would collect the same mass of metal. But it don't, so you collect less mass.

 

[drechie]

But with that reasoning, why were the values for Zinc (2+) greater than Cu(1+). By comparison, Cd (2+) should be greater than Ag(1+)? Where am I seeing this wrong?

Because silver has a charge of +1 and Cd has a charge of +2.

You are moving a fixed number of electrons, therefore you will get half as many Cd atoms as you will silver atoms. Sure if Cd weighed twice as much as silver you would collect the same mass of metal. But it don't, so you collect less mass.

 

[sazerac]

But with that reasoning, why were the values for Zinc (2+) greater than Cu(1+). By comparison, Cd (2+) should be greater than Ag(1+)? Where am I seeing this wrong?

What's the charge on your copper ion again?