Why is dissociation of water important?

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Why is dissociation of water important?

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Answer 1 · 2017-12-31T08:53:54

Well, it underlies our understanding of acid-base behaviour....

Explanation:

Water is just another solvent, admittedly it is a very important solvent, and one that provides the basis for ALL of biochemistry.

In the bulk solvent we conceive of acids, characteristic cations, and base, characteristic anions:

$H_{2} O (l) ⇌ H^{+} + H O^{-}$ $H_2O(l) rightleftharpoonsH^+ + HO^-$

Alternatively.....

$2 H_{2} O (l) ⇌ H_{3} O^{+} + H O^{-}$ $2H_2O(l) rightleftharpoonsH_3O^+ + HO^-$

Just to note that in aqueous solution under standard conditions, the ion product...

$K_{w} = [H_{3} O^{+}] [H O^{-}] = 10^{- 14}$ $K_w=[H_3O^+][HO^-]=10^(-14)$ ...

And we can take ${log}_{10}$ $log_10$ of both sides to give....

${log}_{10} K_{w} = {log}_{10} 10^{- 14} = {log}_{10} [H_{3} O^{+}] + {log}_{10} [H O^{-}]$ $log_(10)K_w=log_(10)10^(-14)=log_10[H_3O^+]+log_10[HO^-]$ .

And thus.... $- 14 = {log}_{10} [H_{3} O^{+}] + {log}_{10} [H O^{-}]$ $-14=log_(10)[H_3O^+]+log_(10)[HO^-]$

Or.....

$14=underbrace(-log_(10)[H_3O^+]-log_(10)[HO^-])_(pH+pOH=14)$

$14=pH+pOH$

This relationship allows us to make very accurate assessments of $pH$ and acidity with relatively simple apparatus. Even with A level equipment, you can get exceptionally accurate and meaningful results in acid-base titrations.

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Why is dissociation of water important?

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