b
\(\frac{{P_{solvent}^0\, - \,\,{P_{solution}}}}{{{P_{solution}}}}\,\, = \,\,\frac{{{n_{solute}}}}{{{n_{solvent}}}}\)

\( \Rightarrow \,\,\,\frac{{640\,\, - \,\,600}}{{600}}\,\, = \,\,\frac{{2.175\,\, \times \,\,78}}{{{{(Mol.wt.)}_{solute}}\,\, \times \,\,39}}\)

\( \Rightarrow \,\,\frac{{40}}{{600}}\,\, = \,\,\frac{{2.175\,\, \times \,2}}{{{{(Mol.wt.)}_{solute}}}}\)

\( \Rightarrow \,\,{(M.wt.)_{solute}}\, = \,\,65.25\,\, \approx \,\,65.3\)