[alanjz]

Could someone help me understand where these answers came from?

Co³⁺+e^- --> Co²⁺ E^O = 1.82 V
Co²⁺+2e^- --> Co E^O = -0.28 V
Given these standard reduction potentials, what is the standard reduction potential for Co³⁺+3e^- --> Co?
I just did 1.82 + -0.28=1.54 but the answer is 0.42 V.

A solution of 0.10 M NaZ has a pH=8.90. What is the K_a of HZ?
The answer was 1.6x10^-5
I really wasn't sure where to start so if an explanation is too much to ask for, at least a hint would be nice.

Thanks.

[Corribus]

Question 1: Electric standard potentials are not additive.  The reason is because adding one electron and then adding two more in separate steps has a different energetic cost than adding all three at the same time.  You need to convert the electric potentials into quantities that ARE additive.  Can you think of what that might be?

Question 2: You should start by writing out some balanced equations or reactions.  For instance - what is Ka for a weak acid?

[alanjz]

Thanks a lot! I think I figured them out.
For 1, I converted them to ΔG, added them, then converted back to standard reduction potentials so (1)(96500)(1.82)+(2)(96500)(-0.28)=121590 then 121590/(3*96500)=0.42
For 2, I just set up the equation for K_b:
[OH^-][HZ]/[Z^-] Solved for K_b then put that under 10^-14 to get K_a