[nobOdy98]

I am having a hard time determining if a reaction will take place or not based on the pKa. For example

Toluic Acid (pKa 3.91) + Sodium Bicarbonate (pKa 6.3) --> Carbonic Acid + Toluic acid Ion

I know that the Toluic acid will be deprotonated into the ion but I don't know why this happens? I'm also expected to know which phase ( Organic or Aqueous) the products will be in.

[Kryolith]

This is an equilibrium. With the pKa values (which ones?) you can determine the equilibrium constant of the reaction.

EDIT:
Assuming that CO₂ will not be removed from the equilibrium.

[nobOdy98]

I realize that you can convert equil. constants but is there a way to determine if the rxn will proceed with just the pKa?

[agrobert]

Yes comparing pK_as will tell you if the reaction will proceed but you need to calculate the extent or equilibrium as already stated.  Look up the pK_a for phenol?  Will sodium bicarbonate deprotonate phenol? No. Why? Look at pK_a and compare to benzoic or toluic acid pK_a.

[AWK]

Simple rule: lower pKa - stronger acid
and
stronger acid replace a weaker one