What is the pKa of water?

What is the pKa of water?

 

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The Ka of water at 25℃ is 1×10^-14 and the pKa is 14.0.

 

In most general chemistry textbooks, the pKa of water at 25℃ is listed as 14.0.

 

In many organic chemistry textbooks and some biochemistry texts, however, the pKa of water at 25℃ is listed as 15.7.

 

This module describes the derivation of these two values and describes why the value of 15.7 should not be used.

 

[ 출처 https://chem.libretexts.org/Core/Organic_Chemistry/Fundamentals/What_is_the_pKa_of_water%3F ]

 

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Water as an Acid

 

Water is both an acid and a base.

It is in equilibrium with solvated protons (acid) and solvated hydroxide (base).

 

For the acid dissociation of water as an acid:

H2O(aq) + H2O(l) ⇌ H3O^+(aq) + OH^-(aq)

or H2O(aq) ⇌ H^+(aq) + OH^-(aq)

 

Ka = [H^+][OH^-] / [H2O]

 

Kw = 10^-14 = [H^+][OH^-]

water density = 1 g/mL = 1000 g/L

1000 g / (18.02 g/mol) = 55.49 mol

[H2O] = 55.49 mol/L

 

Ka = (10^-14) / (55.49) = 1.8×10^-16

pKa = -log(1.8×10^-16) = 15.74