Phosphorite, also called phosphate rock, is a mineral containing PO₄^3– and OH^– anions and Ca²⁺ cations. It is treated with sulfuric acid in the manufacture of phosphate fertilizers (see Chapter 3). A chemist finds the calcium content in an impure sample of phosphate rock by weighing out a 0.4367-g sample, dissolving it in water, and precipitating the Ca²⁺ ions as the insoluble hydrated salt* CaC₂O₄ · H₂O (C₂O₄^2– is called the oxalate ion). After being filtered and dried (at a temperature of about 100°C so that the extraneous water is driven off but not the water of hydration), the CaC₂O₄ · H₂O precipitate weighed 0.2920 g. Calculate the mass percent of calcium in the sample of phosphate rock