[Spiffy Girl]

I'm not sure how to solve this problem. I tried using my basic knowledge of acid/base equilibrium equations but I don't think it was helpful. Here's the question:
If you dissolve caffeine in acid a proton goes onto one of the nitrogens to give the conjugate acid of caffeine, caffineH+. The acid dissociation constant (Ka) of caffeineH+ is Ka=0.25M. If a cup of tea has a pH of 7.6, what should be the ratio of concentration of caffeineH+ to unprotonated caffeine in the tea?
Basically, how do i get pH to Ka?
I know Ka=concentraion of product/concentration of reactants. But I'm only given the pH. Help please?

[kiwi]

I'm not sure how to solve this problem. I tried using my basic knowledge of acid/base equilibrium equations but I don't think it was helpful. Here's the question:
If you dissolve caffeine in acid a proton goes onto one of the nitrogens to give the conjugate acid of caffeine, caffineH+. The acid dissociation constant (Ka) of caffeineH+ is Ka=0.25M. If a cup of tea has a pH of 7.6, what should be the ratio of concentration of caffeineH+ to unprotonated caffeine in the tea?
Basically, how do i get pH to Ka?
I know Ka=concentraion of product/concentration of reactants. But I'm only given the pH. Help please?

So you know about K_a; can you convert that to pK_a? From a pH and a pK_a you can determine the ratio of acid/base in a buffered solution using a very general equation which you will have covered in class. Got any ideas?

[Spiffy Girl]

Ka=[A-][H+]/[HA]
Is it that one?

[kiwi]

Ka=[A-][H+]/[HA]
Is it that one?

Close, does this ring a bell?

http://en.wikipedia.org/wiki/Henderson-Hasselbalch_equation