amino acids & pKa

[cloak25]

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At what pH value will the ratio of the +2 to +1 forms of the molecule be 10:1?

A. 3.0
B. 5.1
C. 7.1
D. 8.2

Answer: B

anyone have a simpler approach to this?

 

[kasho11]

pH = pKa + log [conjugate base/acid]

pH = 6.1 + log [1/10] , log of 0.1 = -1

pH = 6.1 + -1

pH = 5.1

 

[cloak25]

pH = pKa + log [conjugate base/acid]

pH = 6.1 + log [1/10] , log of 0.1 = -1

pH = 6.1 + -1

pH = 5.1

How did you know to use 6.1 in that equation?

 

[kasho11]

Because you want to know the relationship between the +2 and +1 forms. So you start with the +2 version, which is with just the hydroxyl deprotonated, so the next proton to be lost comes from the NH in the ring, pKa 6.1. Going from the +2 to the +1 is entirely based upon that group losing or gaining a proton.

 

[cloak25]

Because you want to know the relationship between the +2 and +1 forms. So you start with the +2 version, which is with just the hydroxyl deprotonated, so the next proton to be lost comes from the NH in the ring, pKa 6.1. Going from the +2 to the +1 is entirely based upon that group losing or gaining a proton.

ahh got it. thanks!