55. Aqueous solutions of AgNO3, Cu(NO3)2 and Cr(NO3)3 are electrolyzed using the same quantity of electricity. How do the number of moles of metal formed compare?
A. Ag = Cu = Cr
B. Ag > Cu > Cr
C. Ag < Cu < Cr
D. Cu > Ag > Cr
The correct answer is B. To answer this question, we need to understand how electrolysis works. In an aqueous electrolytic cell, redox reaction happens so that the cation of the salt or hydrogen ion from water is reduced to its elemental form and the anion or hydroxide from water is oxidized to its oxidation product or oxygen. Whether water will be involved depends on the reduction potentials of the cation and the anion. If the anion has a lower reduction potential than hydroxide, then its oxidation is dominant. If the cation has a higher reduction potential than hydrogen, its reduction is then dominant. Also we need to take into account of the pH and over potential effect of water. In this question, since all of the metal ions have higher reduction potential than hydrogen, they are going to be reduced. As diver ion, cooper ion and chromium ion has a charge of +1, +2 and +3 respectively, to get one mole of metal reduced, they will need 1, 2 and 3 moles of electrons respectively. If the same amount of electricity is consumed, then the mole number of the metals would be: 1mole Ag:0.5mole Cu:0.33mole chromium. Thus, B is the correct answer.
Following is question 56 of the series:
56.
I will post the answer in my next blog.
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