From the sources, it may be found that the heat of solution of potassium nitrate (KNO3) is equal to 34.89 kJ/mol. In order to absorb 120 kJ of heat, the amount of potassium nitrate that should be added to water to form the solution is,
120 kJ = (34.89 kJ/mol)(x)
The value of x is approximately equal to 3.44 mols. This is equal to 347.72 grams of potassium nitrate.