significance of pH=pKa.

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RSK25

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I don't truly understand whats the significance of pH=pKa concept. I understand mathematically that when pH equals to pKa, the ratio of the undissciated acid and its anion is equal to each other. I got confused when i was looking at the following attachment.
 

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As far as I can understand, since the pKa of acetic acid indicates that it is a weak acid, (it does not fully dissociate), the major species in an almost neutral aqueous environment would be the [HA]
 
Yeah, my mistake. Just realized it as well (putting it into Henderson Hasselbalch)
 
I'm inclined to say that the major species of acetic acid in water is the undissociated acid, and if it was on the exam that would be my answer. I am however a bit confused myself why I would get a conflicting answer with the Henderson Hasselbalch equation.

~7=4.75 + [A-][/HA] looks as if [A-] would be the major species
 
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Ahh. I saw my mistake. The pH of that solution would actually be lower than the pKa, not higher.

Using TBR's shortcut: pH= 1/2pKa - 1/2[HA], the pH would be ~2.9

Then using the Hasselbalch equation: 2.9= 4.75 +log[A-]/[HA] You can see that [HA] has to be larger than [A] in order for the sides to equal
 
You don't need to use that specific formula, it's just a shortcut to find the pH of a solution of weak acid

The formula is pH= 1/2pKa -1/2log[HA]
In this case: pH= (4.75/2) -1/2(-1)
pH= ~2.4 +.5
pH=~2.9

This shortcut works if the pKa is from 2-12 and the weak acid concentration must be greater than the Ka. For practical purposes, this is met 99% of the time in these types of questions
 
Oh, i think i get it. it's asking what is the major product after .10 M solution has been prepared. it's poorly worded, i think
 
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