DanielDaniels
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I was wondering about the use of Pka when we compare it to pH.

Now Pka can help us to classify acids in a wider range. So instead of a narrow scale of 0-14, now we can have like (-20) to 60, which is much more helpful Secondly, as it is a relationship, we can use it to find Ka and this will help us to find the concentration of an acid/base/conjugate acid/conjugate base.

Is this right? And what further uses it can provide?
I just want to appreciate the PKa term more and understand why we use it.
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Pigster
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pH doesn't go from 0-14, the record lowest is -31 (I seem to remember).

A similar Q is, "Why do we use pH values? and not just express [H+]?"

The pKa scale, like the pH scale is simpler to state. You know an acid with a pH of 1 is a bit dangerous, whereas pH 5 is safer. A hydrogen ion concentration of 0.1 or 0.00001 is harder to interpret.

Likewise a weak acid with an pKa of 1 is a stronger weak acid than one with a pKa of 5.
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DanielDaniels
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(Original post by Pigster)
pH doesn't go from 0-14, the record lowest is -31 (I seem to remember).

A similar Q is, "Why do we use pH values? and not just express [H+]?"

The pKa scale, like the pH scale is simpler to state. You know an acid with a pH of 1 is a bit dangerous, whereas pH 5 is safer. A hydrogen ion concentration of 0.1 or 0.00001 is harder to interpret.

Likewise a weak acid with an pKa of 1 is a stronger weak acid than one with a pKa of 5.
Exactly. Thanks a bunch for taking part!
I did think about the pH vs [H+] some time earlier.

Now the question is why then we use pKa values when we have pH? What's its most significant advantage?


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Pigster
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pKa expresses the strength of a weak acid, whereas pH simply tells you [H+].

One can have a concentrated weak acid or a dilute stronger acid both with a relatively high pH.

Consider two weak acids: one has a pKa of 1.00 and a concentration of 0.0001 mol dm-3, pH = 2.00; the other has a pKa of 5.00 and a concentration of 10.0 mol dm-3, also with a pH of 2.00.

(I hope)
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DanielDaniels
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(Original post by Pigster)
pKa expresses the strength of a weak acid, whereas pH simply tells you [H+].

One can have a concentrated weak acid or a dilute stronger acid both with a relatively high pH.

Consider two weak acids: one has a pKa of 1.00 and a concentration of 0.0001 mol dm-3, pH = 2.00; the other has a pKa of 5.00 and a concentration of 10.0 mol dm-3, also with a pH of 2.00.

(I hope)
Wow! I really got it. Thanks a ton!


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