How do I do the last part? Thanks! In the refinement of gold a basic solution of

Question

How do I do the last part?
Thanks! In the refinement of gold a basic solution of Au(CN)4- is electrolyzed. Gold metal and oxygen gas are produced at the electrodes. Answer the following questions by selecting the appropriate reaction. Which is the overall cell reaction for this electrolysis? Incorrect: 4Au(CN)4- + 6H2O rightarrow 4Au + 16CN- + 3O2 + 12H+ Incorrect: 4Au + 16CN + 3O2 + 6H2O rightarrow 4Au(CN)4- + 12OH- Correct: 4Au(CN)4- + 12OH- rightarrow 4Au + 16CN- + 3O2 + 6H2O Incorrect: 2Au + 8CN + 6H2O rightarrow 2Au(CN)4- + 6OH- + 3H2 Which half-reaction takes place at the anode during this electrolysis? Incorrect: Au + 4CN- rightarrow Au(CN)4- + 3e- Incorrect: Au(CN)4- + 3e- rightarrow Au + 4CN- Correct: 4OH- rightarrow O2 + 2H2O + 4e- Incorrect: O2 + 2H2O + 4e- rightarrow 4OH- Which half-reaction takes place at the cathode during this electrolysis? Incorrect: O2 + 2H+ + 2e- rightarrow H2O2 Incorrect: Au + 4CN- rightarrow Au(CN)4- + 3e- Incorrect: 4OH- rightarrow O2 + 2H2O + 4e- Correct: Au(CN)4- + 3e- rightarrow Au + 4CN- For the electrolysis described above, what volume of pure oxygen gas is evolved at -6 degree C and 756 Torr for every gram of gold produced.

Explanation / Answer

The over all reaction is 4 Au(CN)4- +12 OH - ---> 4 Au + 16 CN- + 3 O 2 + 6 H2O In the above reaction no . of moles of Oxygen gas liberated ,n = 3 moles We know that PV = nRT Where P = Pressure = 756 torr = 756 / 760 atm                =0.9947 atm           T =Temperature = - 6 oC                                     = -6 + 273 = 267 K           R = gasconstant = 0.0821 L atm / mol - K            n= 3 moles    V = Volume of O2 = ? Plug the above values we get V = nRT / P                                                 = ( 3 * 0.0821 * 267 ) / 0.9947                                                 = 66.11 L

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