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Question about PKA, TPR Organic Book

matth87

Full Member
May 8, 2009
289
1
0
  1. Pre-Medical
p. 746 Question #6

If an amino group has a Ka of 10^-9 what is its Pka.

Ok easy enough its going to be 9.

Part 2

What is the ratio of protenated to unprotenated moieties at equilbrium?
Answer 10^9

I thought the formula for Ka is

Ka = [A-][H+]/[HA]
How come they are not accounting for the concentration of [H+] ions in the solution and are simply saying the ratio of protenated to unprotenated is 10^9?
 

G1SG2

Full Member
10+ Year Member
5+ Year Member
May 2, 2008
1,454
2
0
  1. Pre-Medical
p. 746 Question #6

If an amino group has a Ka of 10^-9 what is its Pka.

Ok easy enough its going to be 9.

Part 2

What is the ratio of protenated to unprotenated moieties at equilbrium?
Answer 10^9

I thought the formula for Ka is

Ka = [A-][H+]/[HA]
How come they are not accounting for the concentration of [H+] ions in the solution and are simply saying the ratio of protenated to unprotenated is 10^9?

I don't see that example in my book (is it the 2009 ed.?). Are you given the pH, or is that all you're given? Because I'm thinking you can use the HH equation?
 

matth87

Full Member
May 8, 2009
289
1
0
  1. Pre-Medical
I don't see that example in my book (is it the 2009 ed.?). Are you given the pH, or is that all you're given? Because I'm thinking you can use the HH equation?

Nope no ph or no H+, its the 2008 edition. If you have the 2009 then I would assume they took the question out because it was bunk.

I think you would have to know an [H+] or pH value to solve this.

If you agree we can call it a day. lol

I have the 2009 edition as well, let me go check in that book...
 
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G1SG2

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10+ Year Member
5+ Year Member
May 2, 2008
1,454
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  1. Pre-Medical
Nope no ph or no H+, its the 2008 edition. If you have the 2009 then I would assume they took the question out because it was bunk.

I think you would have to know an [H+] or pH value to solve this.

If you agree we can call it a day. lol

I have the 2009 edition as well, let me go check in that book...

Yeah, I think so too. Because #6 on pg. 746 in the 2009 BS manual says:

Acetic acid has a pKa=4.7 Calculate the equilibrium ratio of [CH2COO-] to [CH3COOH] at pH 4.7.

In which case, we would just use the HH equation and end up with the ratio=1.
 
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