[bmu123]

A sample of the perovskite SrFeO_x (A) was synthesised by heating SrCO₃ and Fe₂O₃ in oxygen gas at 900°C.  X-ray powder diffraction data revealed a primitive unit cell for A consistent with the perovskite structure.  A small sample of A was dissolved in dilute HCl containing excess KI, and the iodine produced (corresponding to reduction of Fe(IV) to Fe(III)) was titrated with sodium thiosulphate.  The titration indicated that 72% of the iron was present as Fe(IV)

I'm confused about how to work out the composition of A from this, I know 72% of the iron is Fe³⁺ so 28% must be Fe³⁺, this makes the average Fe oxidation state 3.72+, where do I go from here? Would it be SrFeO_2.86 because the overall positive charge is now +5.72 so /2 for number of oxygens needed to balance it out?

X-ray diffraction revealed that A has a primitive unit cell with length 3.898 Å.  Explain the difference between the unit cell size of A and that of SrFeO₃, which has a = 3.852 Å.

I assume this has something to do with the charges but I'm not sure what.

[unsu]

72%, is it a mass % or mole %?

[bmu123]

72%, is it a mass % or mole %?

hmm that's the full question it doesn't say which