On se propose de doser 100 cm^3 d'une solution d'acide chlorhydrique de concentration ca=2 ·10^

step 1 Electrolide, potassium chloride, potassium nitrate, hydrogen chloride, sodium acetate, sodium ch

On se propose de doser 100 cm^3 d'une solution d'acide...

On se propose de doser $100 \mathrm{~cm}^3$ d'une solution d'acide chlorhydrique de concentration $\mathrm{c}_{\mathrm{a}}=2 \cdot 10^{-2} \mathrm{~mol} \mathrm{~L}^{-1}$ par une solution de soude de concentration $\mathrm{c}_{\mathrm{b}}=5.10^{-1} \mathrm{~mol} \mathrm{~L}^{-1}$. a) Est-il nécessaire, au préalable, d'étalonner le conductimètre ? b) Donner l'expression de la conductivité en fonction du volume $V_b$ de base versé : > avant l'équivalence > après l'équivalence. c) Est-il nécessaire de tenir compte de la dilution dans le calcul des concentrations? Exprimer la conductivité en fonction de $V_{\mathrm{b}}$ et tracer la courbe donnant la conductivité $\sigma$ en fonction du volume ajouté $V_{\mathrm{b}}$. Comment peut-on mettre en évidence le point d'équivalence? Données des conductivités ioniques molaires à dilution infinie : $$ \begin{aligned} & \lambda^{\circ}\left(\mathrm{H}^{+}\right)=350 \mathrm{Scm}^2 \mathrm{~mol}^{-1}, \lambda^{\circ}\left(\mathrm{Na}^{+}\right)=50 \mathrm{Scm}^2 \mathrm{~mol}^{-1}, \\ & \lambda^{\circ}\left(\mathrm{Cl}^{-}\right)=76 \mathrm{Scm}^2 \mathrm{~mol}^{-1}, \end{aligned} $$ $\lambda^{\circ}\left(\mathrm{HO}^{-}\right)=198 \mathrm{Sm}^2 \mathrm{~mol}^{-1}$ (on supposera que les conductivités ioniques molaires ne varient pratiquement pas avec la concentration).

On se propose de doser $100 \mathrm{~cm}^3$ d'une solution d'acide chlorhydrique de concentration $\mathrm{c}_{\mathrm{a}}=2 \cdot 10^{-2} \mathrm{~mol} \mathrm{~L}^{-1}$ par une solution de soude de concentration $\mathrm{c}_{\mathrm{b}}=5.10^{-1} \mathrm{~mol} \mathrm{~L}^{-1}$.
a) Est-il nécessaire, au préalable, d'étalonner le conductimètre ?
b) Donner l'expression de la conductivité en fonction du volume $V_b$ de base versé :
> avant l'équivalence
> après l'équivalence.
c) Est-il nécessaire de tenir compte de la dilution dans le calcul des concentrations?
Exprimer la conductivité en fonction de $V_{\mathrm{b}}$ et tracer la courbe donnant la conductivité $\sigma$ en fonction du volume ajouté $V_{\mathrm{b}}$. Comment peut-on mettre en évidence le point d'équivalence?
Données des conductivités ioniques molaires à dilution infinie :
$$
\begin{aligned}
& \lambda^{\circ}\left(\mathrm{H}^{+}\right)=350 \mathrm{Scm}^2 \mathrm{~mol}^{-1}, \lambda^{\circ}\left(\mathrm{Na}^{+}\right)=50 \mathrm{Scm}^2 \mathrm{~mol}^{-1}, \\
& \lambda^{\circ}\left(\mathrm{Cl}^{-}\right)=76 \mathrm{Scm}^2 \mathrm{~mol}^{-1},
\end{aligned}
$$
$\lambda^{\circ}\left(\mathrm{HO}^{-}\right)=198 \mathrm{Sm}^2 \mathrm{~mol}^{-1}$ (on supposera que les conductivités ioniques molaires ne varient pratiquement pas avec la concentration).

Key Concepts

Instrument Calibration

Calibration is the process of verifying and adjusting the accuracy of an instrument's measurements. In the context of conductimetric titrations, ensuring that the conductimeter is properly calibrated is essential for obtaining reliable conductivity data. Calibration accounts for any systematic errors and ensures that the measured values accurately reflect the true conductivity of the solution.

Dilution Effects in Titration

During a titration, the addition of titrant increases the overall volume of the solution, which dilutes the ionic concentrations. Recognizing and accounting for this dilution is important for accurately relating conductivity measurements to actual ion concentrations, ensuring that variations due to changes in volume are properly considered in the analysis.

Equivalence Point Determination

The equivalence point in a titration is reached when the moles of titrant added exactly neutralize the analyte according to its stoichiometry. In conductimetric titrations, the equivalence point is often identified by a distinct change in the conductivity trend, as the nature and concentration of the ionic species in solution undergo abrupt changes.

Acid-Base Neutralization Reaction

Acid-base neutralization is a chemical reaction where an acid reacts with a base to produce water and a salt. The stoichiometry of this reaction is central to titrations, since the point at which the amounts of acid and base are stoichiometrically equivalent—the equivalence point—can be detected through changes in properties such as conductivity.

Ionic Molar Conductivity

Ionic molar conductivity is a measure of how well an ion conducts electricity in solution per unit concentration. It plays a crucial role in calculating the total conductivity of an electrolyte solution, as each ion contributes proportionally based on its specific conductivity and concentration. This concept allows one to predict the conductivity changes during titrations involving ionic species.

Conductimetric Titration

Conductimetric titration involves monitoring the conductivity of a solution during a titration. As a titrant is added, the changing ionic composition alters the solution’s conductivity, which can be used to determine reaction progress and identify key points, such as the equivalence point. This method is particularly useful when the reaction produces ions with significantly different mobilities.

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