Chemistry Problem

Question

A sample weighing 3.148 g is a mixture of Fe2O3 (molar mass = 159.69 g/mol) and Al2O3 (molar mass = 101.96 g/mol). When heat and a stream of H2 gas is applied to the sample, the Fe2O3 reacts to form metallic Fe and H2O(g). The Al2O3 does not react. If the sample residue (the solid species remaining after the reaction) weighs 2.851 g, what is the mass fraction of Fe2O3 in the original sample?

Answer 1

Let f be the mass fraction of Iron(III) oxide (IO) so the mass of IO is 3.148f and the mass of aluminum oxide (AO) is 3.148(1-f)

The mass of AO is unaffected, but the mass of Fe at the end (assuming all IO goes to Fe)

3.148f *( 2(55.85)/159.69) = 2.2012f (You could write the reaction equation and solve for Fe using stoichiometric calculation, but this is much easier)

The final mass balance is

2.2012f + 3.148(1-f) = 2.851 (rearrange algebraically and solve for f)