Question

Arrange the following oxides in correct order of their acidic character. N$_2$O$_5$, P$_2$O$_5$, N$_2$O$_3$, As$_2$O$_3$

• A. I>III>II>IV
• B. III>I>II>IV
• C. IV>II>I>III
• D. III>I>IV>II

Answer 1

The Correct Option is A

Concept: The acidic character of oxides depends on:

1. Oxidation state of the central atom — higher oxidation state generally → more acidic.
2. Electronegativity of the central atom — more electronegative → more acidic.
3. Periodic trend:
   • Across a period: acidity increases (due to increasing electronegativity).
   • Down a group: acidity decreases (due to decreasing electronegativity and increasing size).

Analyzing Given Oxides:

• I. \( \mathrm{N_2O_5} \): Nitrogen in +5 oxidation state, very electronegative → forms strong acid \( \mathrm{HNO_3} \). ✅
• II. \( \mathrm{P_2O_5} \): Phosphorus in +5 oxidation state, less electronegative → forms moderate acid \( \mathrm{H_3PO_4} \).
• III. \( \mathrm{N_2O_3} \): Nitrogen in +3 oxidation state → forms weak acid \( \mathrm{HNO_2} \).
• IV. \( \mathrm{As_2O_3} \): Arsenic in +3 oxidation state, least electronegative → forms very weak acid \( \mathrm{H_3AsO_3} \). 🔽

Stepwise Comparisons:

• \( \mathrm{N_2O_5} > \mathrm{N_2O_3} \) → due to higher oxidation state of nitrogen.
• \( \mathrm{P_2O_5} > \mathrm{N_2O_3} \) → P has higher oxidation state (+5 vs +3), but N is more electronegative. However, acid strength data supports: 
  \( K_a(\mathrm{H_3PO_4}) \approx 7.5 \times 10^{-3} > K_a(\mathrm{HNO_2}) \approx 7.2 \times 10^{-4} \) ⇒ \( \mathrm{P_2O_5} > \mathrm{N_2O_3} \)
• \( \mathrm{N_2O_3} > \mathrm{As_2O_3} \) → N is more electronegative than As.

Correct Order Based on Analysis:

\[ \boxed{\mathrm{N_2O_5} > \mathrm{P_2O_5} > \mathrm{N_2O_3} > \mathrm{As_2O_3}} \quad \text{(I > II > III > IV)} \]

However, Option (a) is: I > III > II > IV

This implies: \( \mathrm{N_2O_5} > \mathrm{N_2O_3} > \mathrm{P_2O_5} > \mathrm{As_2O_3} \), suggesting that the electronegativity of N is more dominant than the higher oxidation state of P.

Conclusion: Based on acid strengths of corresponding oxoacids (real-world data), the more accurate order is:

\[ \boxed{\mathrm{N_2O_5} > \mathrm{P_2O_5} > \mathrm{N_2O_3} > \mathrm{As_2O_3}} \quad \text{(I > II > III > IV)} \]

But the marked answer is: Option (a): I > III > II > IV — which prioritizes electronegativity over oxidation state in II vs III comparison.

Therefore: If we strictly follow acid strength values, option (a) is incorrect. The correct logical order is:

\[ \boxed{\text{I > II > III > IV}} \]

Note: If exam marking indicates (a) as correct, it's likely an oversimplified approach emphasizing electronegativity dominance, which is not supported by acid strength data.