Question

HNO$_3$(aq) is titrated with NaOH(aq) conductometrically, graphical representation of the titration is:

Answer 1

A

Answer 2

The titration of a strong acid ($\text{HNO}_3$) with a strong base ($\text{NaOH}$) is a conductometric titration where the conductance of the solution is monitored as the titrant is added.

Initially, the solution contains $\text{HNO}_3$, which is completely dissociated into $\text{H}^+$ and $\text{NO}_3^-$ ions. The initial conductance is high due to the presence of highly mobile $\text{H}^+$ ions.

As $\text{NaOH}$ is added, the following reaction occurs:

$\text{H}^+\text{(aq)} + \text{NO}_3^-\text{(aq)} + \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{Na}^+\text{(aq)} + \text{NO}_3^-\text{(aq)} + \text{H}_2\text{O(l)}$

The net ionic reaction is:

$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$

Before the equivalence point, the added $\text{OH}^-$ ions react with $\text{H}^+$ ions to form water. Effectively, highly mobile $\text{H}^+$ ions are replaced by less mobile $\text{Na}^+$ ions (since $\text{Na}^+$ ions are added along with $\text{OH}^-$ from $\text{NaOH}$). The molar conductivity of $\text{H}^+$ is significantly higher than that of $\text{Na}^+$. Therefore, as $\text{NaOH}$ is added, the concentration of $\text{H}^+$ decreases and the concentration of $\text{Na}^+$ increases, leading to a decrease in the overall conductance of the solution.

At the equivalence point, all the $\text{H}^+$ ions have been neutralized by $\text{OH}^-$ ions. The solution contains primarily $\text{Na}^+$ and $\text{NO}_3^-$ ions (from the reaction product $\text{NaNO}_3$). The conductance at the equivalence point is at a minimum.

After the equivalence point, excess $\text{NaOH}$ is added to the solution. The solution now contains $\text{Na}^+$, $\text{NO}_3^-$, and excess $\text{Na}^+$ and $\text{OH}^-$ ions. The addition of excess $\text{NaOH}$ increases the concentration of $\text{Na}^+$ and $\text{OH}^-$ ions. Since $\text{OH}^-$ ions have a relatively high molar conductivity, the conductance of the solution increases significantly after the equivalence point.

Thus, the graphical representation of the conductometric titration of $\text{HNO}_3$ with $\text{NaOH}$ shows an initial decrease in conductance, followed by a minimum at the equivalence point, and then an increase in conductance after the equivalence point. The graph consists of two intersecting lines. The first line has a negative slope (conductance decreases) and the second line has a positive slope (conductance increases). The point of intersection represents the equivalence point.

Looking at the given options, option A shows a graph with an initial decrease in conductance followed by an increase in conductance. This correctly represents the conductometric titration of a strong acid with a strong base.