q about pH and pKa!

[drpepper47]

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these are from ochem odyssey
1) Neutral acid has Ka=1.2x10^-5. when pH =9 the acid exists mainly as : an anion.

so, you solve that pKa = 5 which is the point when we deprotonate 50% of the H's. At pH =9, we are well beyond the pKa value. so, the acid exists mainly as X-.

could someone explain this to me please? Why does the acid exist mainly as an anion at pH=9? Thanks!!

2) Consider acetic acid, pKa =4.34. when pH = 2 which is present? answer: neutral acid

the solutions manual says that the hydrogen does not come off until pH=4.34. i thought it was at that pH when 50% was deprotonated? Why is it neutral at pH =2?

 

[ethanyoo726]

For Question 1, the pKa = 5 - log 1.2, which is about 5 (log 1.2 is about 0). This means that at pH of 5, the acid has 50% of its protons deprotonated. At at higher pH of 9, you take away more H+ which is why, your acid exists as an anion (-).

For Question 2, it already gives you the pKa of 4.34, which is where acetic acid has 50% of its H's gone. Acetic acid is already neutral to begin...so, at a lower pH of 2, it should still remain neutral because it should still have most of it's H's.

I don't know if that makes any sense. I hope it helped somewhat.

 

[drpepper47]

thanks ethan!