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the pH of a .100 M solution of sodium formate = 8.37. Calculate the value of Ka of formic acid.

okay so I know that the anion of a weak acid is itself a weak base

so A- +H2O---> OH- + HA

pOH =14-8.37 so conc of OH is 10^-5.63

Ka=[OH][HA]/[A-] so we know the conc of OH and A- and then im stuck helppp

  • chem -

    Convert to (OH^-) = ?? I get 2.34 x 10^-6 but check me out on that. (HA) is the same. (A^-) is 0.1. Solve for Ka.

  • chem -

    I know that part, but why are the conc. of OH and HA the same ?

  • chem -

    When one 1 mole of A^- hydrolyzes, it produces 1 mole of HA and 1 mole of OH^-. Or you can make up an ICE chart from your equation.
    A^- + HOH ==> HA + OH^-
    initial:
    A^- = 0.1.
    HA = 0
    OH = 0

    change:
    HA = +x
    OH^- = +x
    A^- = -x

    equilibrium:
    HA = x
    OH^- = x
    A^- = 0.1-x
    etc.
    Don't hesitate to follow through if you still don't understand.

  • chem -

    OHHHH got it, without the ICE chart I sort of get confused. Thanx!

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