The amount of force is what this is for
Potential energy can be calculated by the formula Pe=mgh. Plug in your values:
Pe=mgh
Pe=(6 kg)(9.8m/s^2)(100 m)
Pe=5880 kg x m^2/s^2, or 5880 Joules
Once the substance stops dissolving, the system is at equlibrium with the water and the undissolved salt now, if it is in the process of dissolving because it is completely soluble but has not been able to completely dissolve, it is not at equilibrium