A sample of iron oxide has FeO and Fe₂O³ in the mole ratio 2:1 . It is partially oxidised to change this ratio to 1:2 . The number of moles of FeO oxidised per mole of initial mixture is?

Initial
Let the Number of moles of FeO be  = 2x mol
Number of moles of Fe₂O₃ = x mol

Final ratio:
Let the Number of moles of FeO be  = y mol
Number of moles of Fe₂O₃ = 2y mol​
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Fe₂O₃ + 2FeO + 1/2O₂ → 2Fe₂O₃

2 moles of  Fe2O3 is produced by = 2 moles of FeO

2y moles of Fe2O3 is produced by  = 2y moles of FeO

Thus
Number of moles of FeO oxidised  = 2y

​also we know 
Initial moles of FeO - Number of Moles reacted = Number of moles present in final mixture

2x-2y = y
​3y = 2x
y =2/3 x

Thus number of moles of FeO oxidised = (2/3)x

Total number of moles of Initial mixture = x + 2x = 3x 

Number of moles of FeO oxidised per mole of Initial mixture = $\frac{2x}{3\left(3x\right)}=\frac{2}{9}$


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