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    Acid-Base Equilibria

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    1. Determining Ka. What is the Ka value for a weak acid if a 0.0150-M solution has a pH of 2.67?

    2. Calculations With Ka. What is the pH of a 0.250-M benzoic acid solution?

    3. Polyprotic Weak Acid. What are pH, [H3O+], [HSO3-], and [SO32-] in a 1.50 M sulfurous acid solution?

    4. Calculations With Kb. What is the Kb value for a weak base if a 0.250-M solution has a pOH of 4.7?

    5. The pH of a Salt Solution. What is the pH of 0.50-M sodium benzoate? The benzoate ion (C7H5O2-) is the anion of benzoic acid.

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    Solution Preview

    For any weak acid HA, we can write its dissociation equation:

    HA --> H+ + A-

    This a chemical equation, and for it we can then make an equilibrium equation, which is simply reactants over the products.

    K = [H+][A-]/[HA]

    In this case we will call the equilibrium a Ka.

    If we were to run what is sometimes called an ICE table fo r the dissciation reaction...

    HA --> H+ + A-
    Initial C 0 0
    Change -x +x +x
    Equilibrium C-x x x

    And if we now substitute these values for concentrations into our Ka expression, we will then get the general equation for a weak acid.

    Ka = x2/C-x

    You can do the same thing for a weak base and arrive at

    Kb ...

    Solution Summary

    For these questions one must first understand that in chemistry, most things are modelled using an equation. The equation you need to use here is quite basic, and learning how to use it will be essential for your performance on exams.