Example 6A.2

Acidity $K_{a}$
Basicity $K_{b}$
The Conjugate Seesaw $K_{a}\times K_{b}=K_{w}$

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Example 6A.2

In example 6A.2 in the textbook, the question is: Barium hydroxide is a base that is sometimes used for titrating acids. When using it, you need to know the actual concentration of hydroxide ion in the solution. What are the molar concentrations of H3O1+ and OH2- in 0.0030 m Ba(OH)2(aq) at 25 C?

In the solution, they stated that the molar concentration of OH2- can be determined by the molarity of Ba(OH)2 molecule (which makes sense), but how can H3O value be determined by dividing the Kw/[OH-]? Or I guess in other words, I don't understand why K value of water is so important when the sample contains another molecule mixed in the solution.

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Re: Example 6A.2

Kw is derived from the equilibrium constant for the autoionization of water into H3O+ and OH2-. Equilibrium constant values are not affected by adding or removing product or reactant. Adding base (or acid) shifts the ratio of reactant species to more IH2-. But Kw which is the product of [H3O+] and [OH2-] will not change, so you can then find the new concentration of H3O+ using Kw