Conceptual Challenge Problems
These rigorous, thought-provoking problems integrate conceptual learning with problem solving and are suitable for group work.
CP19.C (Section 19.11) A piece of chromium metal is attached to a battery and dipped into 50 mL of 0.3-M KOH solution in a 250-mL beaker. A stainless steel electrode is connected to the other electrode of the battery and immersed in the same solution. A steady current of 0.50 A is maintained for exactly 2 hours. Several samples of a gas formed at the stainless steel electrode during the electrolysis are captured, and all are found to ignite in air. After the electrolysis, the chromium electrode is weighed and found to have decreased in weight by 0.321 g. The mass of the stainless steel electrode does not change.
After electrolysis, the KOH solution is neutralized with nitric acid to a pH of slightly less than 7, then is heated and reacted with 0.151-M lead(II) nitrate solution. As the lead(II) nitrate solution is added, a yellow precipitate quickly forms from the hot solution. The formation of precipitate stops after 40.4 mL of the lead(II) nitrate solution has been added. The yellow solid is then filtered, dried, and weighed. Its mass is 1.97 g.
(a) How much electrical charge passes through the cell?
(b) How many moles of Cr react?
(c) What is the oxidation state of the Cr after reacting?
(d) Assuming that the yellow compound that precipitates from the solution during the titration contains both Pb and Cr, what do you conclude to be the ratio of the numbers of atoms of Pb and Cr?
(e) If the yellow compound contains an element other than Pb and Cr, what is it and how much of it is in the compound? What is the formula for the yellow compound?
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