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Dec 19, 2013
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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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Jun 5, 2013
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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]
 
Jun 5, 2013
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Yeah, my mistake. Just realized it as well (putting it into Henderson Hasselbalch)
 
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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
 
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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
 
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Dec 19, 2013
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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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