The resistance of 0.1 M weak acid HA in a conductivity cell is  $2\times {10}^3 \mathrm{Ohm}$. The cell constant of the cell is $0.78 {\mathrm{Cm}}^{-1}$ and  ${\lambda }_{\mathrm{m}}^{\circ }$  of acid HA is  $390 \mathrm{S} {\mathrm{cm}}^2\mathrm{mol}{e}^{-1}\text{.}$ The pH of the solution is