Acid-base equilibria
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When calculating the pH of a strong acid I know OH- are still present, but where have they come from if the acid produces the many H+ and water is acting as a base?
Jenkin replies
The equilibrium H2O <=> H+ + OH- will establish itself in any aqueous solution. You will probably recall that the equilibrium constant for this, Kw, has the value
1 x 10-14 mol2 dm-6 at 298 K. Clearly, the higher the concentration of H+, as in a strong acid, the smaller the concentration of OH-, but because equilibrium is established, [OH-] will never fall to zero.
In pure water, the concentrations of H+ and OH- are equal; they will both be 1 x 10-7 mol dm-3.
In a 1.0 mol dm-3 solution of a strong acid, [H+] = 1.0 mol dm-3, so [OH-] will be
1 x 10-14 mol dm-3.
The origin of the OH- ions can only be water molecules.
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updated: 25 January 2006
