[dun13203171]

I am looking at a question which has the structure of amoxicillin, it asks what % of the compound is ionised at pH3 and it has a pka 2.5.

So using the henderson hasclebalch equation

pH=pKa + log (base/acid) ---> minus pka from both sides

0.5= log (base/acid) ----> take anti log of both sides

3.162/1 =  (base/acid) ------> So am I interpreting this correctly, there will be 3.162 x ionised at pH3?

From this how would I calculate percent ionised?

thanks in advance

[Borek]

You need to know total amount of acid and base together - which you don't know. However, the ratio will work OK if you use molar fractions of both, and that in turn means you can safely assume x_acid+x_base=1.

[dun13203171]

Hi borek thanks for your reply.

I am unsure how to use molar fractions, ive seen this question in a past paper and for that reason I want to ensure I can do it just incase it pops up in an exam.

So using the henderson H. equation, I have found 3.162/1 =  (base/acid) which I believe to be correct?

So thats 3.162 X of the conjugate base whichwill exist pH3. Have I interpreted this correctly?

From this could you show me how to solve for percent ionised? This would be so very much appreciated.

thanks

[Borek]

So thats 3.162 X of the conjugate base whichwill exist pH3.

Hard to comment on not knowing what is X intended to mean.

You know what is the ratio of concentrations. If you have problems with molar fractions, assume total concentration of the acid and conjugate base to be 1M (or any other number, the final answer will be identical, as it cancels out) - from knowing their sum and their ratio, can you find each one separately?