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Using the Henderson hasslebach equation, is my equation correct?

Never used this equation before guys, so thought I would check if im correct.

I have to calculate which form of aspirin predominates at two different pH's - 2 and 8.

Heres my calculations, are they correct

pH = pKa + log [A¯] / [HA]
8.0 = 3.49 + log [A¯]/ [HA]
8.0 - 3.49 = 4.51 = log [A¯]/ [HA]
104.51 = 32359.366 = [A¯]/ [HA]

pH = pKa + log [A¯] / [HA]
2.0 = 3.49 + log [A¯]/ [HA]
2.0 - 3.49 = - 1.49 = log [A¯]/ [HA]
101.49 = 0.03 = [A¯]/ [HA]

How do I explain what the results show?

For pH8, does this mean that at pH8, 32359 molecules of aspirin are in the ionised for for every 1 of the unionised form and using the same reasoning for explaining at pH2?

thanks in advance
(edited 8 years ago)
Original post by hurricaneee
Never used this equation before guys, so thought I would check if im correct.

I have to calculate which form of aspirin predominates at two different pH's - 2 and 8.

Heres my calculations, are they correct

pH = pKa + log [A¯] / [HA]
8.0 = 3.49 + log [A¯]/ [HA]
8.0 - 3.49 = 4.51 = log [A¯]/ [HA]
104.51 = 32359.366 = [A¯]/ [HA]

pH = pKa + log [A¯] / [HA]
2.0 = 3.49 + log [A¯]/ [HA]
2.0 - 3.49 = - 1.49 = log [A¯]/ [HA]
101.49 = 0.03 = [A¯]/ [HA]

How do I explain what the results show?

For pH8, does this mean that at pH8, 32359 molecules of aspirin are in the ionised for for every 1 of the unionised form and using the same reasoning for explaining at pH2?

thanks in advance


Seems like a very complicated way of coming to a decision!

Are you given the pKa of aspirin?

If so, then when [HA] = [A-] then pH = pKa

at any pH lower than this the molecular form predominates and at any pH higher than this the cationic form predominates
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hurricaneee
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Yes charco the Pka of aspirin is 3.49, I just have to do the calculation to put into a paper! It looks ok?

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