Electrochemistry 2 Question 2
2. Which one of the following graphs between molar conductivity $\left(\Lambda_{\mathrm{m}}\right)$ versus $\sqrt{C}$ is correct?
(2019 Main, 10 April II) (a) $\Lambda_{\mathrm{m}}$
(c) $\Lambda$
(b) $\Lambda_{m}$
(d)
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Solution:
- $\mathrm{NaCl}$ and $\mathrm{KCl}$ are strong electrolytes. So, the study of their molar conductances $\left(\lambda_{m}\right)$ can be experimentally verified by Debye-Huckel Onsagar equation,
$\Lambda_{\mathrm{m}}^{c}=\Lambda_{\mathrm{m}}^{0}-B \sqrt{C}$
$\Lambda_{\mathrm{m}}^{c}=$ molar conductance at concentration.
$\Lambda_{\mathrm{m}}^{0}=$ molar conductance at infinite dilution. i.e. $C \rightarrow 0$
$B=$ Debye-Huckel Onsagar constant.
For (both $\mathrm{NaCl}$ and $\mathrm{KCl}$ ) a strong binary electrolyte like $A B$, the nature of the plot of $\Lambda_{\mathrm{m}} v s \sqrt{C}$ will be
Size of $\mathrm{Na}^{\oplus}$ is being smaller than $\mathrm{K}^{\oplus}$ and $\mathrm{Na}^{\oplus}$ will remain in more hydrated state, i.e. larger sized in aqueous solution. As a result, ionic mobility as well as ionic conductance of $\mathrm{Na}^{\oplus}$ (or $\mathrm{NaCl}{\text {as }} \mathrm{Cl}^{\ominus}$ is common to $\mathrm{NaCl}$ and $\mathrm{KCl}$ ) will be lower than $\mathrm{K}^{\oplus}$ (or $\mathrm{KCl}$ ). Thus, the plot of $\Lambda{\mathrm{m}} v s \sqrt{C}$ for $\mathrm{NaCl}$ and $\mathrm{KCl}$ is as follows :
