During the course of an experiment with hydorgen phsophate (HPO42-) you find that it is functioning as a base. When you search for the Kb value of it you only find pKa1= 2.15, pKa2= 7.20, and pKa3= 12.35 of phosphoric acid (H3PO4). What is the Kb value??

pKa+pKb=14

if it is a base, we need to find the highest pKb
pKb=14-12.35=1.65
kb=10^-1.65