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 (in kg) has to dissolve in water to absorb 100 kJ of heat?

Answer 1

when you dissolve potassium nitrate , you imagine it being pulled apart from a gas of seperate ions, then those seperate ions all are hydrated.

so, to pull it apart the cost is 163.8kcal for every mole of potassium nitrate.

Then when you hydrate it, you get back 155.5kcal. so, overall the change has cost 163.8-155.5kcal for every mole. so, if it is 8.3 kcal/mol and a mole is 101gms and if we calculate in a kcal yhen it is is 4200J.

So, we can say that 100gms absorbs 34860J of energy.

Finally 34.86KJ or each KJ comes from 2.897gms .

so,you have need to calculate for 100KJ of heat then it is 323gms of potassium nitrate.