Given the heat of formation of the solid salt is -131.81kj/mol and the molecular mass of the solid salt is 107.55g/mol calculate the heat of formation of the solution

What you actually need is the heat of lattice of the salt and the heat of hydration of the salt. With heat of formation, you cannot easily get the heat of solution, unless you have other information.

\Delta H_{solution} = \Delta H_{hydration} - \Delta H_{lattice}