Challenge Problems 119. Three electrochemical cells were connected in series so b. Hydroxide ion interferes with the measurement of F. Therefore, the response of a fluoride electrode is that the same quantity of electrical current passes through all three cells. In the first cell, 1.15 g of chromium metal was deposited from a chromium(III) nitrate solution. In the second cell, 3.15 g of osmium was deposited from a solution made of Os*+ and nitrate ions. What is the name of the salt? In the third cell, the electrical charge passed through a solution containing X2+ ions caused deposi- tion of 2.11 g of metallic X. Identify X. Emeas = Et - 0.05916 log([F-] + k[OH-]) where k = 1.00 x 10' and is called the selectivity factor for the electrode response. Calculate [F-] for the data in part a if the pH is 9.00. What is the per- cent error introduced in the [F-] if the hydroxide interference is ignored? c. For the [F-] in part b, what is the maximum pH such that [F-/k[OH-] = 50.? d. At low pH, F- is mostly converted to HF. The fluo- ride electrode does not respond to HF. What is the minimum pH at which 99% of the fluoride is pres- ent as F- and only 1% is present as HF? e. Buffering agents are added to solutions containing fluoride before making measurements with a fluoride-selective electrode. Why? 120. An electrochemical cell is set up using the following unbalanced reaction: N2+(aq) + M(s) M**(aq) + N(s) The standard reduction potentials are Ma+ + ae – g° = +0.400 V N2+ + 2e- g° = +0.240 V The cell contains 0.10 M N2+ and produces a voltage of 0.180 V. If the concentration of M-+ is such that the value of the reaction quotient Q is 9.32 × 10-3, calcu- late [M*+]. Calculate wmax for this electrochemical cell. 123. When copper reacts with nitric acid, a mixture of NO(g) and NO2(g) is evolved. The volume ratio of the two product gases depends on the concentration of the nitric acid according to the equilibrium 121. A zinc-copper battery is constructed as follows: 2H*(aq) + 2NO;-(aq) + NO(g) = 3NO2(g) + H2O(1) Zn|Zn²*(0.10 M)||Cu²*(2.50 M)|Cu Consider the following standard reduction potentials at The mass of each electrode is 200. g. a. Calculate the cell potential when this battery is first 25°C: NO(g) + 2H2O(1) 3e- + 4H*(aq) + NO3-(aq) connected. b. Calculate the cell potential after 10.0 A of current has flowed for 10.0 h. (Assume each half-cell con- tains 1.00 L of solution.) c. Calculate the mass of each electrode after 10.0 h. d. How long can this battery deliver a current of 10.0 A before it goes dead? E° = 0.957 V e- + 2H*(aq) + NO; (aq) → NO2(g) + H,O(1) E° = 0.775 V a. Calculate the equilibrium constant for this reaction. b. What concentration of nitric acid will produce an NO and NO, mixture with only 0.20% NO, (by moles) at 25°C and 1.00 atm? Assume that no other gases are present and that the change in acid con- centration can be neglected. 122. The measurement of F ion concentration by ion- selective electrodes at 25.00°C obeys the equation Emcas = Eret – 0.05916 log[F¯] a. For a given solution, Emcas is 0.4462 V. If E,f is 0.2420 V, what is the concentration of F- in the solution?