If the molar conductivity (${\Lambda }_m$) of a $0.050{\text{mol L}}^{-1}$ solution of a monobasic weak acid is $90{\text{S cm}}^2{\text{mol}}^{-1}$, its extent (degree) of dissociation will be [Assume ${\lambda }_{+}^{\circ }=349.6{\text{S cm}}^2{\text{mol}}^{-1}$ and ${\lambda }_{-}^{\circ }=50.4{\text{S cm}}^2{\text{mol}}^{-1}$]

Solution & Explanation

\textbf{Answer:} (A) $\alpha =0.225$ \textbf{Solution:} Limiting molar conductivity by Kohlrausch's law: ${\Lambda }_m^{\circ }={\lambda }_{+}^{\circ }+{\lambda }_{-}^{\circ }=349.6+50.4=400{\text{S cm}}^2{\text{mol}}^{-1}$ Degree of dissociation: $\alpha =\frac{{\Lambda }_m}{{\Lambda }_m^{\circ }}=\frac{90}{400}=0.225$