# chem

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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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