There is no formula for the molar heat of solution.
You have to do an experiment to determine the heat of solution of a substance.
Experimentally, you dissolve a known amount of the substance in a given mass of the solvent and measure the temperature change.
Then you use the specific heat of the solution and the number of moles of the solute to calculate the molar heat of solution.
What is the molar heat of ammonium nitrate if 8.00 g of NH₄NO₃ dissolved in 50.0 g of water in a coffee cup calorimeter causes the temperature to drop by 10.1 °C? Assume that the specific heat capacity of the solution is 4.18 J·°C⁻¹g⁻¹.
There are two heats involved.
heat of solution of NH₄NO₃ + heat from water = 0