From the given diagram, 6 mL volume of HA used till equivalence point. At half of equivalence point, solution will be basic buffer with B and BH+.pOH=pKb​+log[B][BH+]​At half equivalence point : [BH+]=[B](∵pH=11)Therefore, pOH=pKb​=14−11=3pKb​=3.00