The molarity (M) of a solution is calculated by dividing the number of moles of solute by the volume of the solution in liters. In this case, we have 0.400 mol of HCl in 9.79 L of solution.

$$Molarity (M) = \frac{Moles \ of \ Solute}{Volume \ of \ Solution \ (L)}$$

$$Molarity (M) = \frac{0.400 \ mol}{9.79 \ L}= 0.0410 \ M$$

Therefore, the molarity of the solution is 0.0410 M.