Pyrite is a mineral with formula FeS2. An impure sample of pyrite, known to be 90-95% w/w FeS2, is to be analyzed by oxidizing all the sulfur to SO42-

Question:



Pyrite is a mineral with formula FeS2. An impure sample of pyrite, known to be 90-95%

w/w FeS2, is to be analyzed by oxidizing all the sulfur to SO42-, and then precipitating as BaSO4.

Exactly how many grams of the sample must be taken if a minimum of 1.0 gram of BaSO4 is

desired? ( stoichiometry).

Answer:

First we can calculate the number of moles BaSO4:

m( BaSO4 ) 1.0 g
n( BaSO4 )    0.0043mol
M ( BaSO4 ) 233 g / mol

Next we can write the reaction equations:

4FeS2 + 11O2 → 8SO2 + 2Fe2O3

4 mol 8 mol

3SO2 → 2SO3

3 mol 2 mol

SO3 + H2O → H2SO4

1 mol 1 mol

H2SO4 + Ba2+ → BaSO4 + 2H+

1 mol 1 mol

From this:

n( FeS 2 )  0.00286mol

The mass of the FeS2 will be:

m( FeS 2 )  n( FeS 2 )  M ( FeS 2 )  0.00286mol  120 g / mol  0.3432 g