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Electrochemistry
Kohlrausch's Law
7 questions, each with its answer.
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The molar conductance of NaCl, HCl and $CH_3COONa$ at infinite dilution are 126.45, 426.16 and 91.0 S cm$^2$ mol$^{-1}$ respectively. The molar conductance of $CH_3COOH$ at infinite dilution is. Choose the right option for your answer.By Kohlrausch's law: $\Lambda^\circ_{m(CH_3COOH)} = \Lambda^\circ_{m(CH_3COONa)} + \Lambda^\circ_{m(HCl)} - \Lambda^\circ_{m(NaCl)}$ $= 91.0 + 426.16 - 126.45 = 390.71\ S\ cm^2\ mol^{-1}$
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Following limiting molar conductivities are given as $\lambda^0_{m(H_2SO_4)}=x$ S cm$^2$ mol$^{-1}$, $\lambda^0_{m(K_2SO_4)}=y$ S cm$^2$ mol$^{-1}$, $\lambda^0_{m(CH_3COOK)}=z$ S cm$^2$ mol$^{-1}$ $\lambda^0_m$ (in S cm$^2$ mol$^{-1}$) for $CH_3COOH$ will be:By Kohlrausch's law, $\lambda^0_{CH_3COOH}=\lambda^0_{CH_3COO^-}+\lambda^0_{H^+}=\lambda^0_{CH_3COOK}+\frac{1}{2}\lambda^0_{H_2SO_4}-\frac{1}{2}\lambda^0_{K_2SO_4}=z+\frac{x}{2}-\frac{y}{2}=\frac{(x-y)}{2}+z$.
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Limiting molar conductivity of $NH_4OH$ (i.e. $\Lambda_m^\circ(NH_4OH)$) is equal to:By Kohlrausch's law, $\Lambda_m^\circ(NH_4OH) = \lambda^\circ(NH_4^+) + \lambda^\circ(OH^-)$
$\Lambda_m^\circ(NH_4Cl) = \lambda^\circ(NH_4^+) + \lambda^\circ(Cl^-)$
$\Lambda_m^\circ(NaOH) = \lambda^\circ(Na^+) + \lambda^\circ(OH^-)$
$\Lambda_m^\circ(NaCl) = \lambda^\circ(Na^+) + \lambda^\circ(Cl^-)$
Adding the first two and subtracting the third cancels $Na^+$ and $Cl^-$:
$\Lambda_m^\circ(NH_4OH) = \Lambda_m^\circ(NH_4Cl) + \Lambda_m^\circ(NaOH) - \Lambda_m^\circ(NaCl)$ -
Molar conductivities ($\Lambda_m^0$) at infinite dilution of NaCl, HCl and $CH_3COONa$ are 126.4, 425.9 and 91.0 S $cm^2$ $mol^{-1}$ respectively. $\Lambda_m^0$ for $CH_3COOH$ will be:By Kohlrausch's law: $\Lambda_m^0(CH_3COOH) = \Lambda_m^0(CH_3COONa) + \Lambda_m^0(HCl) - \Lambda_m^0(NaCl)$
(The $Na^+$ and $Cl^-$ contributions cancel, leaving $CH_3COO^-$ and $H^+$.)
$= 91.0 + 425.9 - 126.4 = 390.5$ S $cm^2$ $mol^{-1}$ -
Which of the following expressions correctly represents the equivalent conductance at infinite dilution of $Al_2(SO_4)_3$? Given that $\Lambda^\circ_{Al^{3+}}$ and $\Lambda^\circ_{SO_4^{2-}}$ are the equivalent conductances at infinite dilution of the respective ions.Kohlrausch's law: at infinite dilution each ion makes its own contribution to the conductance, independent of the other ion.
In terms of equivalent conductances, one equivalent of any electrolyte contains one equivalent of the cation and one equivalent of the anion.
So $\Lambda^\circ_{eq}(\text{electrolyte}) = \Lambda^\circ_{eq}(\text{cation}) + \Lambda^\circ_{eq}(\text{anion})$, with no stoichiometric multipliers.
The ionic values given are already equivalent conductances, so: $\Lambda^\circ_{eq}[Al_2(SO_4)_3] = \Lambda^\circ_{Al^{3+}} + \Lambda^\circ_{SO_4^{2-}}$
(The form $2\Lambda^\circ + 3\Lambda^\circ$ would apply only if molar ionic conductances were given and the molar conductance were asked.) -
Kohlrausch's Law states that at :Kohlrausch's law of independent migration of ions: at infinite dilution, when dissociation is complete, each ion makes a definite contribution to the equivalent (molar) conductance of an electrolyte, whatever be the nature of the other ion of the electrolyte.
$\Lambda_m^\infty = \lambda_+^\infty + \lambda_-^\infty$
Here $\lambda_+^\infty$ and $\lambda_-^\infty$ are the ionic conductances at infinite dilution for the cation and the anion respectively.
The law holds only at infinite dilution, because only then are the ions far apart and free from inter-ionic attractions; so the statement with 'finite dilution' is wrong.
The contribution of an ion is independent of the other ion, so the statement 'depending on the nature of the other ion' is wrong.
The contribution is to the equivalent (or molar) conductance, not simply to the conductance.
The law is used to find $\Lambda^\infty$ of weak electrolytes such as acetic acid. -
At infinite dilution equivalent conductances of $Ba^{2+}$ and $Cl^-$ ions are 127 and 76 ohm$^{-1}$ cm$^{2}$ eq$^{-1}$ respectively. Equivalent conductance of $BaCl_2$ at infinite dilution is :By Kohlrausch's law, the limiting equivalent conductance of an electrolyte is the sum of the limiting equivalent conductances of its ions.
The value 127 for barium corresponds to the whole $Ba^{2+}$ ion; per equivalent it is $\dfrac{1}{2} \times 127 = 63.5$.
$\Lambda^\circ_{eq}(BaCl_2) = \lambda^\circ\left(\dfrac{1}{2}Ba^{2+}\right) + \lambda^\circ(Cl^-) = 63.5 + 76$
$= 139.5$ ohm$^{-1}$ cm$^2$ eq$^{-1}$
That is every question in Kohlrausch's Law.
Next: Conductance of Electrolytic Solutions – Ostwald's Dilution
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