An aqueous solution is prepared by dissolving 0.1 mol of an ionic salt in 1.8 kg of water at $35^{\circ} \mathrm{C}$. The salt remains $90 \%$ dissociated in the solution. The vapour pressure of the solution is 59.724 mm of Hg . Vapor pressure of water at $35^{\circ} \mathrm{C}$ is 60.000 mm of Hg . The number of ions present per formula unit of the ionic salt is $\_\_\_\_$ .