[oArctico]

I need some help with this particular problem I'm going over for a final, so here it is.

A mixture of .250 moles of an unknown weak acid and 10.0 mL of 3.00 moles NaOH is diluted to 1200mL. The pH was observed to be 5.60. What is the Ka for the weak acid?

So far, I decided to use this equation by converting everything to moles first.

HA + OH^-  H₂O + A^-

where the molarity for OH^- was 2.723 M OH^- and for A^- it was 0.207 M A^-

After that I'm decided to use this equation using the concentrations, but I can't seem to remember how to connect to Ka using

HA + H₂O  H₃O⁺ + A^-

[Borek]

Many ways of skinning that cat, but first you need to have a correct input data.

HA + OH^-  H₂O + A^-

OK

where the molarity for OH^- was 2.723 M OH^- and for A^- it was 0.207 M A^-

Think it over. You forgot you mixed and diluted. Besides, OH^- reacted, so it is no longer present in the solution (well, it is, but in negligible amounts).

After that I'm decided to use this equation using the concentrations, but I can't seem to remember how to connect to Ka using

HA + H₂O  H₃O⁺ + A^-

Write Ka definition, see what you can plug into.