Q5(c). Magnetite from an ignited iron oxide residue

An ore containing magnetite, Fe₃O₄, was analyzed by dissolving a 1.500 g sample in conc. HCl giving a mixture of Fe²⁺ and Fe³⁺. After adding HNO₃ to oxidize any Fe²⁺ to Fe³⁺, the resulting solution was diluted with water and Fe³⁺ precipitated as Fe(OH)₃ by adding NH₃. After filtering and rinsing, the residue was ignited giving 0.8525 g of pure Fe₂O₃. Calculate the % w/w Fe₃O₄ in the solution. [Given: FW of Fe₂O₃ = 159.69 g/mol and FW of Fe₃O₄ = 231.54 g/mol]