Potassium nitrate has a lattice energy of1638 kcalmol and a

Potassium nitrate has a lattice energy of-163.8 kcal/mol and a heat of hydration of -155.5 kcal/mol. How much potassium nitrate has to dissolve in water to absorb 108kJ of heat? Express your answer using two significant figures.

Solution

lattice energy = -163.8 Kcal/mol = -687.96 KJ/mol

heat of hydration = -155.5 Kcal/mol = -653.1 KJ/mol

to absorb 108 KJ heatÂÂÂ

108 = (687.96-653.1)*n

n = 3.098 moles

mass  = 3.098*101.1 = 313.2 g = 0.313 Kg