In a solvent S, a compound B is partially dissociated into C and D as given below:

$$\mathbf{B}\rightleftharpoons 2\mathbf{C}+2\mathbf{D}$$

B, C and D are non-volatile in nature. The molar mass of B is 10 times the molar mass of S. The standard boiling point and the standard enthalpy of vaporization of S are 400 K and $$10R$$ J mol$$^{-1}$$, respectively ($$R$$ is the gas constant in J K$$^{-1}$$ mol$$^{-1}$$). A solution of B in S with an initial concentration of B as 0.25% (mass/mass) has a boiling point of 408 K at 1 bar pressure. In this solution, the mole percent of B that has been dissociated is ___.