Chemistry

Asolution contains a weak monoprotic acid HA and
its sodium salt NaA both at 0.1 M concentration.
Show that [OH�] Kw/Ka.

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  1. Start with an equation

    HA <-> H+ + A-

    at start
    0.1.....0....0

    at equilibrium

    0.1-x...x....x+0.1

    Ka=[H+][A-]/HA
    =x.(0.1+x)/(0.1-x)

    if x is small with respect 0.1 then this expression becomes

    Ka=x.(0.1)/(0.1)
    or Ka=x

    Kw=[H+](OH-]
    =x[OH-]

    so

    [OH-]=Kw/x

    as x=Ka then

    [OH-]=Kw/Ka

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