Potassium hydrogen phthalate, often abbreviated KHP, can be...

Potassium hydrogen phthalate, often abbreviated KHP, can be obtained in high purity and is used to determine the concentrations of solutions of strong bases. Strong bases react with the hydrogen phthalate ion as follows: $$ \mathrm{HP}^{-}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(t)+\mathrm{P}^{2-}(a q) $$ The molar mass of KHP is $204.2 \mathrm{~g} / \mathrm{mol}$ and $K_{a}$ for the $\mathrm{HP}^{-}$ ion is $3.1 \times 10^{-6} .$ (a) If a titration experiment begins with $0.4885 \mathrm{~g}$ of KHP and has a final volume of about $100 \mathrm{~mL}$, which indicator from Figure $16.7$ would be most appropriate? (b) If the titration required $38.55 \mathrm{~mL}$ of $\mathrm{NaOH}$ solution to reach the end point, what is the concentration of the $\mathrm{NaOH}$ solution?

Potassium hydrogen phthalate, often abbreviated KHP, can be obtained in high purity and is used to determine the concentrations of solutions of strong bases. Strong bases react with the hydrogen phthalate ion as follows:
$$
\mathrm{HP}^{-}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(t)+\mathrm{P}^{2-}(a q)
$$
The molar mass of KHP is $204.2 \mathrm{~g} / \mathrm{mol}$ and $K_{a}$ for the $\mathrm{HP}^{-}$ ion is $3.1 \times 10^{-6} .$ (a) If a titration experiment begins with $0.4885 \mathrm{~g}$ of KHP and has a final volume of about $100 \mathrm{~mL}$, which indicator from Figure $16.7$ would be most appropriate? (b) If the titration required $38.55 \mathrm{~mL}$ of $\mathrm{NaOH}$ solution to reach the end point, what is the concentration of the $\mathrm{NaOH}$ solution?

Key Concepts

Acid–Base Titration

Acid–base titration is an analytical technique used to determine the concentration of an acid or base in a solution by gradually adding a titrant of known concentration until the reaction reaches its endpoint. In these titrations, one reactant is often in slight excess and the exact point where the amount of titrant added is stoichiometrically equivalent to the substance being titrated is known as the equivalence point.

Weak Acid–Strong Base Reaction

When a weak acid, such as the hydrogen phthalate ion, reacts with a strong base, the neutralization reaction typically produces water and the conjugate base of the weak acid. The presence of a weak acid means the equivalence point will not necessarily occur at a pH of 7 because the conjugate base can hydrolyze to produce a basic solution, making pH calculations and indicator selection more critical.

Indicator Selection

Indicator selection in titrations is based on the expected pH range of the equivalence point. An appropriate indicator undergoes a distinct color change within a narrow pH range that ideally brackets the equivalence point pH. For a titration involving a weak acid and a strong base, the endpoint tends to be basic, so an indicator with a color change in the higher pH range is most appropriate.

Stoichiometric Calculations

Stoichiometric calculations involve converting the mass of a substance into moles using its molar mass, and then using the balanced chemical equation to relate the moles of reactants at the equivalence point. This process allows the determination of the unknown concentration of a titrant, such as the sodium hydroxide solution in a titration experiment.

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