I recently went through the majority of the olympiad questions and wasn't really stumped or surprised by any of the answers, but when I looked at the most recent math-based questions I felt the writers really increased the difficulty. Just curious someone could give feed back on to how to solve a few problems.http://www.acs.org/content/dam/acsorg/education/students/highschool/olympiad/pastexams/2015-usnco-local-exam.pdf
35. Copper (II) hydroxide, Cu(OH)2
, has a Ksp
. For the reaction below, Keq
. What is Kf
(s) + 4NH3
(aq) + 2OH-
The answer is 1.8x1013
. I tired starting with Ksp and finding the OH- concentration, then tried to use Keq = [Cu(NH3
but this didn't lead to anything. I tried to flip my products and reactants for Keq, but this also didn't lead me to the correct answer. I'm missing a step some where.
The second one was question 41, which asks...
The reduction of O2
O in acidic solution has a standard reduction potential of +1.23 V. What is the effect on the half-cell potential at 25ºC when the pH of the solution is increased by one unit?
(g) + 4H+
(aq) + e-
The answer is A. the half-cell potential decreases by 59 mV
I thought this one was going to be pretty easy by using the Nerst equation. Since the pH is increasing by a unit I set K to / being that as the pH goes up the concentration of H+
would be decreasing by a factor of 10. My equation was E = 1.23 - (0.0592/4 e-
) log(1/10). Nope, that didn't work. Tried to play with numbers and did log(10/1). I knew it wouldn't work, but I tried it anyway and of course it didn't work.
Any suggestions on how to fix, or is there someone that has posted work on how to solve these two problems? Thanks!