JEE Main 2025 — Salt Analysis Question with Solution

Mohr's salt is chemically represented as ${\mathrm{FeSO}}_4\cdot {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\cdot 6{\mathrm{H}}_2\mathrm{O}$.

To understand its composition, Mohr's salt dissociates into the following ions:

Fe${}^{2+}$ (ferrous ion)

2 NH${}_4^{+}$ (ammonium ions)

2 SO${}_4^{2-}$ (sulfate ions)

Thus, Mohr's salt contains ferrous, ammonium, and sulfate ions, but also has water molecules that hydrate these ions.

Using Kohlrausch's Law of independent ionic mobility, the molar conductance at infinite dilution for an electrolyte is the sum of the ionic conductances of its constituent ions:

${\lambda }_{\mathrm{m}}^{\infty }(\text{Mohr’s salt})={\mathrm{x}}_1+2{\mathrm{x}}_2+2{\mathrm{x}}_3$

Here:

$x_1$ represents the molar conductance of the ferrous ion (Fe${}^{2+}$),

$x_2$ represents the molar conductance of the ammonium ion (NH${}_4^{+}$),

$x_3$ represents the molar conductance of the sulfate ion (SO${}_4^{2-}$).

Therefore, statement II should include the correct expression $x_1+2x_2+2x_3$ to accurately describe the molar conductance for Mohr's salt.