Nernst equation

[Hiara]

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I tried to do these two exercises but I didn't get the answer correct so I don't know what to do then! 🙁

1. If the Ksp of copper (1)bromide is 4.2x10^-8, compute the concentration of bromide necessary to cause precipitation in an electrochemical cell with the Cu|Cu+ and H+|H2 couples. Assume the conditions are as follows: E°red of Cu+= 0.521V, pH=0, PH2= 1atm, T=298K; Ecell when precipitation begins=0.82V. Answer: [Br-]= 3.7x10^-13

2. Compute the equilibrium constant at 298K for the cell comprised of the Zn2+| Zn (E°red=-0.76V) and MnO4-|Mn2+ (E°red=1.49V) couples. Answer: Keq=e^876

 

[pabloyikes]

I tried to do these two exercises but I didn't get the answer correct so I don't know what to do then! 🙁

1. If the Ksp of copper (1)bromide is 4.2x10^-8, compute the concentration of bromide necessary to cause precipitation in an electrochemical cell with the Cu|Cu+ and H+|H2 couples. Assume the conditions are as follows: E°red of Cu+= 0.521V, pH=0, PH2= 1atm, T=298K; Ecell when precipitation begins=0.82V. Answer: [Br-]= 3.7x10^-13

2. Compute the equilibrium constant at 298K for the cell comprised of the Zn2+| Zn (E°red=-0.76V) and MnO4-|Mn2+ (E°red=1.49V) couples. Answer: Keq=e^876

Just curious where this question came from. I've seen various opinions as to whether Nernst type questions are likely to be on the MCAT, and I haven't come across one yet.