AIIMS 2005 Chemistry Structure of Compounds MCQ Question

To solve the question regarding the number of P–O–P bridges in the structures of phosphorus pentoxide (P₂O₅) and phosphorus trioxide (P₄O₁₀), we need to analyze the molecular structures of these compounds.

Explanation of the Structures

1. Phosphorus Pentoxide (P₂O₅):

   • The molecular structure of phosphorus pentoxide can be represented in a dimeric form, where two phosphorus atoms are bonded by oxygen atoms. The structure can be visualized as a cyclic arrangement with the formula:
     • Each phosphorus atom is bonded to four oxygen atoms, and two of these oxygen atoms are bridging between the phosphorus atoms.
   • In a complete molecular structure, there are 6 P–O–P bridges formed because:
     • Each phosphorus atom in the dimer contributes to the bridging oxygens, leading to the formation of several P–O–P connections.
   • The complete structural representation of P₂O₅ shows that there are 6 bridges in total.

2. Phosphorus Trioxide (P₄O₁₀):

   • Phosphorus trioxide can be seen as a polymeric structure consisting of four phosphorus atoms. Each phosphorus atom is connected to multiple oxygen atoms.
   • In the structure, there are 5 P–O–P bridges formed. This is because:
     • The phosphorus atoms share oxygen atoms, but due to the three-dimensional arrangement, two phosphorus atoms are not directly bridged, leading to fewer bridging connections.

Summary of the Number of Bridges

• For phosphorus pentoxide (P₂O₅), the number of P–O–P bridges = 6.
• For phosphorus trioxide (P₄O₁₀), the number of P–O–P bridges = 5.

Conclusion

Thus, the correct answer to the question is:

A) (a) 6, 6

Clarification of Other Options

• Option B (5, 5): Incorrect, as it underestimates the number of P–O–P bridges in phosphorus pentoxide (it has 6).
• Option C (6, 5): Incorrect because while it correctly identifies the number of bridges in phosphorus trioxide as 5, it mistakenly states the number for phosphorus pentoxide as 6 (both should be 6).
• Option D (5, 6): Incorrect, as it reverses the numbers for the two compounds.

Therefore, the explanation above confirms that the correct answer is indeed (a) 6, 6 for phosphorus pentoxide and phosphorus trioxide, respectively.