 The phosphate ion has a molar mass of 94.97 g/mol, and consists of a central phosphorus atom surrounded by four oxygen atoms in a tetrahedral arrangement.It is the conjugate base of the hydrogen phosphate ion , which in turn is the conjugate base of the dihydrogen phosphate ion , which in turn is the conjugate base of orthophosphoric acid, .Many phosphates are not soluble in water at standard temperature and pressure.