Rault's law: the vapor pressure of a non-volatile solution is equal to the vapor pressure of the solvent times the molar fraction of the solvent (at the same temperature).
Pv water in the solution = X water * Pv of pure water
745 mmHg = Xw * 760 mmHg
Xw = 745/760 = 0.98
Molar fraction of the solute = 1 - 0.98 = 0.02
Answer: 0.02