One mole of water being in equilibrium with a negligible amount of its saturated vapour at a

Question:

One mole of water being in equilibrium with a negligible amount of its saturated vapour at a temperature T1 was completely converted into saturated vapour at a temperature T2. Find the entropy increment of the system. The vapour is assumed to be an ideal gas the specific volume of the liquid is negligible in comparison with that of the vapour.