ductivity measurements of pure water, is 1.8 X 10 at 25 °C. Substituting this value for Keq in Equation 2-4 gives the value of the ion product of water:

Kw = [H+][OH-] = (55.5 m)(1.8 X 10-16 m) = 1.0 X 10-14 m2

Thus the product [H+][OH-] in aqueous solutions at 25 °C always equals 1 X 10-14 m2. When there are exactly equal concentrations of H+ and OH-, as in pure water, the solution is said to be at neutral pH. At this pH, the concentration of H+ and OH- can be calculated from the ion product of water as follows:

As the ion product of water is constant, whenever [H+] is greater than 1 X 10-7 m, [OH-] must become less than 1 X 10-7 m, and vice versa. When [H+] is very high, as in a solution of hydrochloric acid, [OH-] must be very low. From the ion product of water we can calculate [H+] if we know [OH-], and vice versa (Box 2-2).

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