Sodium chromate is obtained by the fusion of chromite ore, FeCr2O4, with sodium carbonate in free access of air. The unbalanced equation is:
\( \ce{FeCr2O4 + Na2CO3 + O2 -> Na2CrO4 + Fe2O3 + CO2} \)
How many moles of O2 are needed to convert 2 mol of FeCr2O4 completely into the products?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 3.5 mol
Oxygen of the air is the oxidising agent: it oxidises both the iron and the chromium of chromite. The moles of O2 follow from balancing the electrons lost and gained.
In FeCr2O4, Fe is +2 and Cr is +3: (+2) + 2(+3) + 4(−2) = 0.
In the products, Fe is +3 (Fe2O3) and Cr is +6 (Na2CrO4).
Electrons lost per FeCr2O4 = 1 (Fe) + 2 × 3 (Cr) = 7.
Each O2 molecule gains 4 electrons (0 → −2 for two O atoms).
O2 needed per mole of FeCr2O4 = 7/4 mol; this gives the balanced equation below.
For 2 mol of FeCr2O4: 2 × 7/4 = 3.5 mol of O2.
\( \displaystyle \ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2} \)
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “…Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows: 4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2”