Chromite ore, one of the few sources for pure chromium metal, is a mixture of pr

Question

Chromite ore, one of the few sources for pure chromium metal, is a mixture of predominantly Fe and Cr oxides. A 7.10 g sample of chromite was mixed with sodium carbonate and heated in a high O2 environment to produce sodium chromate and insoluble iron oxide. These were separated by dissolving the sodium chromate in water and filtering out the iron oxide. The chromate ions were then recovered by adding Al(NO3)3 to produce a stable aluminum chromate, Al2(CrO4)3, precipitate, which was filtered, dried, and weighed. If 5.82 g of precipitate was produced, what is the mass percent of chromium oxide, Cr2O3, in the original sample of chromite ore? Chromite ore, one of the few sources for pure chromium metal, is a mixture of predominantly Fe and Cr oxides. A 7.10 g sample of chromite was mixed with sodium carbonate and heated in a high O2 environment to produce sodium chromate and insoluble iron oxide. These were separated by dissolving the sodium chromate in water and filtering out the iron oxide. The chromate ions were then recovered by adding Al(NO3)3 to produce a stable aluminum chromate, Al2(CrO4)3, precipitate, which was filtered, dried, and weighed. If 5.82 g of precipitate was produced, what is the mass percent of chromium oxide, Cr2O3, in the original sample of chromite ore? Chromite ore, one of the few sources for pure chromium metal, is a mixture of predominantly Fe and Cr oxides. A 7.10 g sample of chromite was mixed with sodium carbonate and heated in a high O2 environment to produce sodium chromate and insoluble iron oxide. These were separated by dissolving the sodium chromate in water and filtering out the iron oxide. The chromate ions were then recovered by adding Al(NO3)3 to produce a stable aluminum chromate, Al2(CrO4)3, precipitate, which was filtered, dried, and weighed. If 5.82 g of precipitate was produced, what is the mass percent of chromium oxide, Cr2O3, in the original sample of chromite ore?

Explanation / Answer

Chromite ore = 7.10 g

mass of Al2(CrO4)3 = 5.82 g

moles of Al2(CrO4)3 = 5.82 g/401.9442 g/mol = 0.0145 mol

moles of CrO4^2- = 3 x 0.0145 mol = 0.0435 mol

moles of Na2CrO4 from which it is made = 0.0435 mol

moles of Cr2O3 in the sample = 0.0435 mol/2 = 0.02175 mol

mass of chromium oxide (Cr2O3) in the original chromite sample = 0.02175 mol x 151.99 g/mol = 3.306 g

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