Question

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

Question:

How much potassium nitrate has to dissolve in water to absorb 109 kJ of heat? There are 4.184 kJ/kcal. Also note the answer is to be reported in kg.

Answer 1

First convert kJ to kcal as follows:

1 kJ = 4.184 Kcal
109 kJ = 456.056 kcal

Lattice energy of -163.8 kcal/mol is absorbed to break the bonds in the lattice of Potassium nitrate and heat of hydration of -155.5 kcal/mol.is released as the KNO3 is hydrated.

As here lattice energy is absorbed and energy of hydration is released, the net result ia calculate as follows:

Lattice energy - heat of hydration
+ 163.8 – 155.5 = 8.3Kcal / mole

Now calculate the number of mole of potassium nitrate as follows:

8.3Kcal / mole * x moles = 456.056 kcal


Moles = 54.9 mole of KNO3
Now calculate the amount of KNO3 by its molar mass as follows:

Molar mass of KNO3 = 101.1g/mole

101.1g/mole * 54.9 moles =5550.39 g of KNO3

1000 g = 1.0 kg

5550.9 g of KNO3 = 5.55 Kg of KNO3