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?

Explanation / Answer

Chromite is the oxide ore of Chromium. Sometimes it consists of Mg also. Collectively (Fe,Mg) Cr2O4 is referred as Chromite.

The general chemical formula of Chromite is -------------- FeCr2O4

Molecular weight of Chromite is------------------------224g

Chemical reaction between Chromite and Na2CO3 in presence of excess O2 is as shown below:

(Cr2O3 ) Chromite + 2 Na2CO3 + 3/2 O2 2 Na2CrO4 + 2 CO2 -------------------------(1)

224g of chromite produces------------------- 324g of Na2CrO4

7.1g of Chromite produces ---------------- ?

= 10.2g of Na2CrO4

Chemical reaction between sodium chromate and aluminium nitrate

3 Na2CrO4 +2 Al(NO3)3 ----------------> Al2(CrO4)3 + 6 NaNO3 -------------------------------(2)

from the above equation we can get the information as:

486g of Na2CrO4 produces--------------402g of Al2(CrO4)3

10.2 g of Na2CrO4 produces -------------- ?

= 8.43g of Al2(CrO4)3

But it is given in the question that we are getting 5.82g of Al2(CrO4)3

That means 8.43- 5.82 = 2.61g is being removed in the form of iron oxide.

Now mass percentage of Cr2O3 in the sample is :

5.82/8.43 *100 = 69% of Cr2O3 is present in Chromite ore.

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