Number of Co^3+ ions per 100 oxide ions in salt Co_0.94O_1.00 will be-

The correct answer is Option (1) → 12

In a metal deficiency defect the solid has less metal than the ideal formula. Electrical neutrality is kept because some metal ions are in a higher oxidation state. In Co_0.94O_1.00, a few \( \ce{Co^{3+}} \) ions make up for the missing cobalt.

Let x of the 0.94 cobalt ions be \( \ce{Co^{3+}} \); the rest, (0.94 − x), are \( \ce{Co^{2+}} \). The positive charge must balance the charge of one \( \ce{O^{2-}} \) ion:

    \( 3x + 2(0.94 - x) = 2 \)

    \( x + 1.88 = 2 \;\Rightarrow\; x = 0.12 \)

So there are 0.12 \( \ce{Co^{3+}} \) ions per oxide ion, i.e. 12 Co³⁺ ions per 100 oxide ions.

Shortcut: every missing \( \ce{Co^{2+}} \) leaves a charge of 2+ to be made up, and each \( \ce{Co^{3+}} \) supplies one extra positive charge. 0.06 Co is missing per O, so \( \ce{Co^{3+}} \) = 2 × 0.06 = 0.12 per O, i.e. 12 per 100 O.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Non-Stoichiometric Defects – “(ii) Metal Deficiency Defect There are many solids which are difficult to prepare in the stoichiometric composition and contain less amount of the metal as compared to the stoichiometric proportion. A typical example of this type is FeO which is mostly found with a composition of Fe_0.95O. It may actually range from Fe_0.93O to Fe_0.96O. In crystals of FeO some Fe²⁺ cations are missing and the loss of positive charge is made up by the presence of required number of Fe³⁺ ions.”