How much potassium nitrate has to dissolve in water to absorb 110 kj of heat?

Solution:

Potassium nitrate has a lattice energy of -163.8 kcal/mol and a heat of hydration of -155.5 kcal/mol.

Therefore,

Latice Energy: K⁺(g) + NO₃⁻(g) → KNO₃(s) . . . ∆H = –163.8 kcal/mol  

Hydration:  

K⁺(g) + NO₃⁻(g) + excess H₂O → K⁺(aq) + NO₃⁻(aq) . . . ∆H = –155.5 kcal/mol  

Flip the first equation and add them together:  

KNO₃(s) + excess H₂O → K⁺(aq) + NO₃⁻(aq) . . . ∆H = +8.3 kcal/mol = +34. 73 kJ/mol  

(110kJ)/(34. 73 kJ/mol) = 3.16729 mol must be dissolved to absorb 110 kJ