JEE Main: Acidic Oxides + Primary Valency Trick | Inorganic Combo Question 💥

❓ Question

• ‘X’ is the number of acidic oxides among: VO₂, V₂O₃, CrO₃, V₂O₅, Mn₂O₇.

• The primary valency of cobalt in the complex
  $[ \mathrm{Co}(\mathrm{H_2NCH_2CH_2NH_2})_3 ]_2(\mathrm{SO_4})_3$
  (i.e. $[Co(\text{en})_3]_2(SO_4)_3$) is ‘Y’.

Find the value of X + Y.

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🖼️ Question Image

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✍️ Short Solution

We solve in two parts — first count acidic oxides (X), then find the primary valency (Y).

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🔹 Part A — Identify acidic oxides (find X)

For each oxide determine the oxidation state of the metal and typical acid/base character:

1. VO₂ — V oxidation state: +4. Oxides of transition metals at intermediate oxidation states (like +2, +3, +4) are typically basic or amphoteric, not strongly acidic. VO₂ is amphoteric/neutral → not counted as acidic.

2. V₂O₃ — V oxidation state: +3. Lower oxidation oxides are generally basic (or amphoteric) → not acidic.

3. CrO₃ — Cr oxidation state: +6. Chromium(VI) oxide is the acid anhydride of chromic acid (strongly acidic). → acidic.

4. V₂O₅ — V oxidation state: +5. Vanadium(V) oxide is an anhydride of vanadic acids — behaves acidic. → acidic.

5. Mn₂O₇ — Mn oxidation state: +7. Manganese(VII) oxide is highly acidic (anion-forming; permanganate chemistry). → acidic.

So acidic ones are CrO₃, V₂O₅, Mn₂O₇ → total X = 3.

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🔹 Part B — Primary valency of Co in $[Co(\mathrm{en})_3]_2(SO_4)_3$ (find Y)

Interpret the salt: $[Co(en)_3]_2(SO_4)_3$.

• Sulfate ion has charge $-2$. Three sulfates contribute total charge $-6$.

• Two complex cations balance that, so total positive charge = $+6$. Therefore each complex cation has charge $+3$.

• Each cation is $[Co(en)_3]^{3+}$. Ethylenediamine (en) is a neutral bidentate ligand, so it does not change oxidation state of Co.

• Thus cobalt oxidation state in the complex = +3.

In classical coordination terminology the primary valency (ionizable valency) is the oxidation state. So Y = 3.

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🧮 Final Calculation

$$X + Y = 3 + 3 = \boxed{6}$$

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✅ Conclusion & Quick Notes

• Acidic oxides are typically those of higher oxidation states which act as acid anhydrides (CrO₃, V₂O₅, Mn₂O₇ here).

• Primary valency = oxidation state; for $[Co(en)_3]^{3+}$ it is +3.

• Final answer: $\boxed{6}$.