Question

A sample weighing 3.049 g is a mixture of Fe2O3 (molar mass = 159.69) and Al2O3 (molar mass = 101.96). 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 2.425 g, what is the mass fraction of Fe2O3 in the original sample?

Answer 1

The reaction will be

Fe2O3 + 3H2 --> 2Fe   + 3H2O

so here the weight loss is due to only the removal of oxygen from the solid Fe2O3 to give Fe .

The weight loss = 3.049 - 2.425 = 0.624 grams

We can calculate the moles of O removed from this value

Moles = Mass / Atomic weight of O = 0.624 / 16 = 0.039

With 3 moles of O are removed it shows that one mole of Fe2O3 was present

so when 0.039 moles of O are removed it shows the moles of Fe2O3 present = 0.039 / 3 = 0.013 moles

1 mole of Fe2O3 weighs = 159.69 grams

So 0.013 mole will weigh = 2.076 grams

So out of the give sample = 3.049 the mass of Fe2O3 = 2.076 grams

so the mass fraction of Fe2O3 = 2.076 / 3.049 = 0.68 Or 68%