Given below are two statements:
Statement I: Mohr's salt is composed of only three types of ions—ferrous, ammonium, and sulphate.
Statement II: If the molar conductance at infinite dilution of ferrous, ammonium, and sulphate ions are $ x_1 $, $ x_2 $, and $ x_3 $ $ \text{S cm}^2 \, \text{mol}^{-1} $, respectively, then the molar conductance for Mohr's salt solution at infinite dilution would be given by $ x_1 + x_2 + 2x_3 $.

Question

Rods $x$ and $y$ of equal dimensions but of different materials are joined as shown in figure. Temperatures of end points $A$ and $F$ are maintained at $100^\circ$C and $40^\circ$C respectively. Given the thermal conductivity of rod $x$ is three times of that of rod $y$, the temperature at junction points $B$ and $E$ are (close to):

Answer 1

Step 1: Analysis of Statement I
Mohr's salt comprises three ions: ferrous ($ \text{Fe}^{2+} $), ammonium ($ \text{NH}_4^+ $), and sulphate ($ \text{SO}_4^{2-} $).
This statement is accurate as Mohr's salt, also known as ammonium ferrous sulphate, exclusively contains these three ionic species.
Consequently, Statement I is correct.
Step 2: Analysis of Statement II
The molar conductance of a salt at infinite dilution equals the sum of the molar conductances of its constituent ions. For Mohr's salt, the relevant ions are:
Ferrous ion ($ \text{Fe}^{2+} $) with molar conductance $ x_1 $, Ammonium ion ($ \text{NH}_4^+ $) with molar conductance $ x_2 $, and Sulphate ion ($ \text{SO}_4^{2-} $) with molar conductance $ x_3 $. At infinite dilution, the total molar conductance ($ \lambda_{\infty} $) of Mohr's salt should be the sum of the conductances of these ions. However, Statement II inaccurately assigns a coefficient to the sulphate ion. The sulphate ion ($ \text{SO}_4^{2-} $) is a monoatomic ion and should contribute $ x_3 $ to the total conductance, not $ 2x_3 $.
Thus, the correct formulation for molar conductance is: \[ \lambda_{\infty} = x_1 + x_2 + x_3 \]
Therefore, Statement II is erroneous due to the incorrect doubling of the sulphate ion's contribution.

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The Correct Option is C

Solution and Explanation

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