$\left[\mathrm{KHP}\right]\mathbf{=}\frac{\mathbf{0}\mathbf{.}\mathbf{5}\mathbf{}\overline{)\mathbf{g}\mathbf{}\mathbf{KHP}}\left({\displaystyle \frac{1\mathrm{mol}\mathrm{KHP}}{204.22\overline{)g\mathrm{KHP}}}}\right)}{\mathbf{100}\mathbf{}\overline{)\mathbf{mL}}\left({\displaystyle \frac{{10}^{-3}L}{1\overline{)\mathrm{mL}}}}\right)}\mathbf{=}$**0.0245 M**

${\mathbf{K}}_{\mathbf{a}}\mathbf{=}{\mathbf{10}}^{\mathbf{-}{\mathbf{pK}}_{\mathbf{a}}}\mathbf{=}{\mathbf{10}}^{\mathbf{-}\mathbf{5}\mathbf{.}\mathbf{51}}\mathbf{=}$**3.09×10**^{-6}

${\mathbf{K}}_{\mathbf{b}}\mathbf{=}\frac{{\mathbf{K}}_{\mathbf{w}}}{{\mathbf{K}}_{\mathbf{a}}}\mathbf{=}\frac{\mathbf{1}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{14}}}{\mathbf{3}\mathbf{.}\mathbf{09}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{6}}}\mathbf{=}$**3.24×10 ^{-9}**

Reaction: HP^{- }_{(aq)} + OH^{-}_{(aq)} → H_{2}O_{(l)} + P^{2-}_{(aq)}

Construct an ICE Chart:

$\overline{){{\mathbf{K}}}_{{\mathbf{b}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}{\mathbf{=}}\frac{\left[{\mathrm{HP}}^{-}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[{P}^{2-}\right]}}$

Potassium hydrogen phthalate, known as KHP (molar mass = 204.22 g/mol), can be obtained in high purity and is used to determine the concentration of solutions of strong bases by the reaction

HP^{- }_{(aq)} + OH^{-}_{(aq)} → H_{2}O_{(l)} + P^{2-}_{(aq)}

If a typical titration experiment begins with approximately 0.5 g KHP and has a final volume of about 100 mL, what is an appropriate indicator to use? The pK_{a} for HP ^{-} is 5.51.

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