Question

How many moles of NH4NO3(s) must be dissolved in water so that 73.0 kJ of heat is absorbed from the water? (The molar heat of solution of NH4NO3 is 25.7 kJ/mol.)

Answer 1

Answer: 2.84 moles

Explanation: Heat of solution is the heat change associated with the dissolution of a substance in a solvent at constant pressure.

The enthalpy of solution is most often expressed in kJ/mol at constant temperature.

It is given that: Molar heat of solutionof $NH_4NO_3$ = 25.7 kJ/mol

25.7 kJ of heat is absorbed when 1 mole of $NH_4NO_3$ is dissolved in water.

73.0 kJ is released when=$\frac{1}{25.7}\times 73.0=2.84moles$ are dissolved in water.

Answer 2

1 mole of NH4NO3 requires absorption of 25.7 kJ/mol.
Therefore; 1 kJ of heat energy is absorbed by dissolving 1/25.7 moles of NH4NO3. 
Hence; For an energy of 73.0 kJ to be absorbed the moles of NH4NO3 that dissolves will be;
 = (1/25.7 × 73)
 = 2.84 moles
Hence; 73 kJ is absorbed by dissolving 2.84 moles of NH4NO3.