Fresh water is to be extracted at a rate of 100 L/s from brackish water at 15oC

Question:

Fresh water is to be extracted at a rate of 100 L/s from brackish water at 15oC with a salinity of 0.05% (by mass). Determine
(a) The mole fraction of water in the brackish water,
(b) The minimum power (in kW) required, and
(c) The minimum pressure to which the brackish water must be pumped if the fresh water is to be obtained by reverse osmosis using semipermeable membrane.