What is the relationship between pKa & pH?

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Global Warming

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I'm a little confused...could anyone shed some light on this?

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pKa is a measure of acid strength. It depends on the identity and chemical properties of the acid.

pH is a measure of [H+] in a solution.

So, if you have a 1 M soln of acid A (pKa = 5) and a 1 M soln of acid B (pKa = 20), soln with acid A will have a lower pH than the soln with acid B.
 
keq= [A-][H+]/([HA][H20])

Since water is the solvent and exists in a very large proportion compared to [H+] and the acid/conjugate base; it has a nearly constant molarity and we can redefine the terms a little bit.

ka= keq*[(some water molarity constant)] = [A-][H+]/[HA]

we take the -log of both sides

-log(ka) = -log([A-][H+]/[HA]), -log(ka) is the definition of pka

pka= -log([A-][H+]/[HA]) = log([HA]/[A-][H+]) = log([HA]/[A-]) - log([H+]) by property of logs

-log([H+]) is the definition of ph

pka = ph + log([HA]/[A-]) Which is just an analogue of what jm192 said

This relationship is called the Henderson Hasselbalch equation
 
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