0.01 M aqueous solutions of following salts were prepared. Their conductivity values were measured
at 25°C. Which of the following will possess highest conductivity at 25°C?

CsCl

The correct answer is option 4. CsCl.

To determine which salt (\(LiCl\), \(NaCl\), \(RbCl\), or \(CsCl\)) will possess the highest conductivity in a \(0.01 M\) aqueous solution at 25°C, we need to understand how conductivity in ionic solutions is influenced by the mobility of ions and the size of cations.

Factors Influencing Conductivity:

Nature of the Ions:

Conductivity in an aqueous solution is primarily determined by the movement of ions. The faster the ions move, the higher the conductivity.

For a given salt, the anion \((Cl^-)\) is the same in all cases. So, the differences in conductivity are due to the varying cation (\(Li^+\), \(Na^+\), \(Rb^+\), \(Cs^+\)).

Ionic Mobility:

Ionic mobility refers to how fast an ion moves in the electric field of the solution. Higher ionic mobility means higher conductivity.

For alkali metal ions (\(Li^+\), \(Na^+\), \(Rb^+\), \(Cs^+\)) ionic mobility increases as the size of the ion increases.

Hydration of Ions:

Smaller cations like \(Li\) are highly hydrated in aqueous solutions because they strongly attract water molecules due to their high charge density (charge per unit size). These cations are surrounded by a large hydration shell (water molecules bound to the ion), which makes them bulkier and slows their movement through the solution.

Larger cations like \(Cs^+\) have lower charge density, so they are less hydrated, and as a result, they can move more freely in solution with less resistance from the surrounding water molecules.

Size and Mobility of Alkali Metal Cations:

Alkali metal ions increase in size down the group from lithium to cesium in the periodic table:

\(Li^+ < Na^+ < Rb^+ < Cs^+\).

As the size of the cation increases, its hydration decreases, leading to higher mobility in solution. Therefore, \(Cs^+\), being the largest cation, will move faster and have higher mobility than (\(Rb^+\), \(Na^+\) and \(Li^+\)\))

Trend of Conductivity Based on Ionic Mobility:

\(LiCl\): \(Li^+\) is the smallest alkali metal ion with the highest charge density. It is heavily hydrated, forming a large hydration shell. This makes the ion effectively larger and slows its movement through the solution, resulting in lower conductivity.

\(NaCl\): \(Na^+\) is larger than \(Li^+\) and has a smaller hydration shell. Therefore, it has higher mobility and higher conductivity compared to \(Li^+\).

\(RbCl\): \(Rb^+\) is larger than \(Na^+\), with even less hydration, leading to higher mobility and higher conductivity than both \(Na^+\) and \(Li^+\)

\(CsCl\): \(Cs^+\) is the largest alkali metal ion with the lowest hydration. It can move through the solution with the least resistance, making it the most mobile ion. Thus, \(Cs^+\) contributes to the highest conductivity among the given options.

Experimental Evidence:

In practice, it has been observed that conductivity values for \(0.01 M\) aqueous solutions of alkali metal chlorides increase as you move down the group from \(LiCl\) to \(CsCl\), with \(CsCl\) showing the highest conductivity due to the high mobility of the \(Cs^+\) ion.

Conclusion:

The larger the cation, the less hydrated it is, leading to higher ionic mobility and, consequently, higher conductivity.

Among the given salts, \(CsCl\) has the highest conductivity due to the largest size and lowest hydration of \(Cs^+\), making it the most mobile ion in solution.

Thus, \(CsCl\) will have the highest conductivity at 25°C.