One of the notable features of the transition element is the great variety of oxidation states it may show in its compounds. The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example exhibits all the oxidation states from +2 to +7. The lesser number of oxidation states at the extreme ends is either due to the loss or sharing of a few electrons (Sc, Ti) or too many d electrons of higher valence. Early in the first series scandium (II) is virtually unknown and titanium (IV) which is more stable than Ti (III) or Ti (II). On the other end, the only oxidation state of zinc is +2.
For balancing the given reaction, value of \(x\) will be:
\(4FeCr_2O_4 + xNa_2CO_3 + 7O_2 \longrightarrow xNa_2CrO_4 + 2Fe_2O_3 + 8CO_2\)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 8
Potassium and sodium dichromate are prepared from chromite ore, \( \ce{FeCr2O4} \). The ore is fused with sodium carbonate in free access of air; chromium is oxidised from +3 to +6 and appears as sodium chromate, while iron ends up as \( \ce{Fe2O3} \).
Balancing atom by atom, starting from the 4 \( \ce{FeCr2O4} \) already given:
Cr: 4 × 2 = 8 on the left, so 8 \( \ce{Na2CrO4} \) on the right.
Na: 8 × 2 = 16 on the right, so 8 \( \ce{Na2CO3} \) on the left.
C: 8 on the left, 8 \( \ce{CO2} \) on the right — balanced.
O: 16 + 24 + 14 = 54 on the left; 32 + 6 + 16 = 54 on the right — balanced.
\( \ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2} \)
So x = 8. The same coefficient stands before \( \ce{Na2CO3} \) and \( \ce{Na2CrO4} \), as the question requires.
Shortcut: the 8 Cr atoms in 4 \( \ce{FeCr2O4} \) must end up in x \( \ce{Na2CrO4} \), so x = 8 at once.
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Potassium Dichromate – “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”