How to calculate the heat of dissolution from a calorimeter experiment?
You have multiplied the mass of the sample, 1.50g, by temperature change and heat capacity. 

However, the water provides most of the heat for the reaction.  

The total mass of the solution is 1.50g + 35.0g = 36.5g.  

You should be multiplying  36.5g by the temperature change and heat capacity.

Then, you need to consider how many moles 1.50g KCl is. 
Divide the change in enthalpy of the solution by the number of moles of KCl to determine the molar heat of solution of KCl.