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