Calculating the Kb value of an unknown base?

Question

A sample of an ionic compoud NaA, where A- is the anion of a weak acid, was dissolved in enough water to make 100 mL of solution and was then titrated with .100 M HCl. After 500 mL of HCl was added, the pH was found to be 5.00. 1.00 L of .100 M HCl was required to reach the stoichiometric point of the titration.

So...
What's the Kb value of A-?

I tried constructing an Ice table and working packwards, but found Kb to be 100,000, so I definitely messed something up. Any help is appreciated, and best answer gets the points. Thanks!

Answer 1

USe Henderson's eqn.

Kb = 2x10^-5