# significance of pH=pKa.

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#### RSK25

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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#### IlyaR

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]

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#### RSK25

but, the pH of a neutral solution is 7 which is higher than pKa

#### IlyaR

Yeah, my mistake. Just realized it as well (putting it into Henderson Hasselbalch)

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#### RSK25

so? sorry i still don't understand.

#### IlyaR

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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#### IlyaR

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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#### RSK25

Using TBR's shortcut: pH= 1/2pKa - 1/2[HA], the pH would be ~2.9
I don't know what this is

#### IlyaR

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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#### RSK25

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

#### IlyaR

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
That's correct, yep!

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