Resistance of 0.2 M solution of an electrolyte is 50 $\Omega$. The specific conductance of the solution is 1.4 S m$^{-1}$. The resistance of 0.5 M solution of the same electrolyte is 280 $\Omega$. The molar conductivity of 0.5 M solution of the electrolyte in S m$^2$ mol$^{-1}$ is: