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The Autoionization of Water

Also called The Autodissociation of Water

Proton exchane between two water molecules

H₂O + H₂O H₃O⁺ + :OH^-

= 3.25x10^-18
K_W = K_eq[H₂O]² = [H₃O⁺][:OH^-]

K_W = [H₃O⁺][:OH^-] = 10^-14 M²

K_W is the autodissociation equilibrium constant for Water
in relation to Hydronium (acid) and Hydroxide (base) concentrations

What is the concentration of Water ?

Use the density of water:

D_H2O = 1.00 g/mL

And the inverse of the molar mass of water:

MM_H2O = 18.015 g/mol
1/MM_H2O = 1 mol/18.015 g

One liter of water is equal to 1000 milliters



1 L_H2O = 1000 mL_H2O

Multiply the above quantities together, cancelling units as you go



(1000mL_H2O) X
(1.00g_H2O/mL_H2O) X
(1mol_H2O/18.0152g_H2O)

= 55.5 mols of H₂O in one Liter

The Molarity of Water in aqueous solutions is always assumed to be:

[H₂O] = 55.5 M

What is the value for K_W?

The Autodissociation of Water

H₂O + H₂O H₃O⁺ + :OH^-

= 3.25x10^-18
K_W = K_eq[H₂O]²
K_W = (3.25x10^-18)[55.5 M]² = [H₃O⁺][:OH^-]
K_W = [H₃O⁺][:OH^-] = 10^-14

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