Determine number of sucrose molecules in the solution

When a solid is dissolved in a volatile solvent the vapour pressure of the solution is less than the vapour pressure of the pure solvent because of the presence of solute molecules.  In a solution, the number of solvent molecules at the surface is reduced and therefore the rate of evaporation is less than for the solvent.  The extent of lowering is proportional to the number of molecules of solvent compared with the total number of solvent plus solute molecules. For e.g. when equal quantities of sucrose and sodium chloride are dissolved in a known amount of water at constant temperature, the lowering of the vapour pressure of water by sodium chloride is twice as much as that of sucrose, because sodium chloride contains twice as many of number of icons  as the number of sucrose molecules in the solution.