Answer
Phenolphthalein
Work Step by Step
$\dfrac{0.5g}{204.22g/mol} = 2.45 \cdot 10^{-3}$ mol
$/dfrac{2.45 \cdot 10^{-3} mol}{0.100L} = 2.45 \cdot 10^{-2}$ M $P^{2-}$
$K_b = \dfrac{1.0 \cdot 10^{-14}}{10^{-5.51}} = 3.24 \cdot 10^{-9}$
$K_b = \dfrac{[HP^-][OH^-]}{[P^{2-}]}$
$3.24 \cdot 10^{-9} = \dfrac {(x){x}}{2.45 \cdot 10^{-2}}$
$7.92 \cdot 10^{-11} = x^2$
$x = [OH^-] = 8.90 \cdot 10^{-6}$ M $OH^-$
$pOH = -log(8.90 \cdot 10^{-6}) = 5.051$
pH = 14 - pOH
pH = 8.949
pKa of Phenolphthalein = 9.3
pH at the equivalence point = 8.9
Phenolphthalein is the best choice.