Question

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 102 kJ of heat? There are 4.184 kJ/kcal. Also note the answer is to be reported in kg.

Answer 1

Lattice energy H_lattice = - 163.8 kcal / mol

Heat of hydration H_hydration = - 155.5 kcal / mol

H_solution = - H_lattice + H_hydration

= 163.8 - 155.5

= 8.3 kcal / mol

= 8.3 * 4.184 kJ / mol

= 34.7272 kJ / mol

Let n mole potassium nitrate is required to dissolve in water to absorb 109 kJ of heat.

n * 34.7272 = 109

n = 3.1387 moles

1 mole potassium nitrate = 101.1032 g

0.08158 moles potassium nitrate = 101.1032 * 3.1387

= 317.332 g

= 0.3173 kg

0.3173 kg  potassium nitrate is required to dissolve in water to absorb 109 kJ of heat.