A and B are completely soluble in the liquid state, but there is no solid solubility. Assume that the solution is a regular solution and derive an expression for the activity at mole fraction xA of the solvent at a temperature T above the liquidus temperature. Take hA as the molar heat of fusion and TA the melting temperature of pure A, and Tc as the liquid temperature at mole fraction xA. The liquid copper-gold alloy behaves as a regular solution. If Cu=0.35 at 1100C, calculate hCu, the heat of mixing for copper