Question

Salt A + S $\xrightarrow{BaCl_2}$ B White ppt.

A is paramagnetic in nature and contains about 55% K. Thus, A is :

• A. K₂O
• B. K₂O₂
• C. KO₂
• D. K₂SO₄

Answer 1

Answer: (C)

KO₂

Answer 2

The compound A is identified based on its properties: paramagnetism and percentage of potassium.

1. Percentage of Potassium: The molar mass of K is approximately 39.1 g/mol, and O is 16.0 g/mol.

   • K₂O: Molar mass $\approx$ 94.2 g/mol. %K $\approx$ (2 $\times$ 39.1 / 94.2) $\times$ 100 $\approx$ 83.0%.
   • K₂O₂: Molar mass $\approx$ 110.2 g/mol. %K $\approx$ (2 $\times$ 39.1 / 110.2) $\times$ 100 $\approx$ 71.0%.
   • KO₂: Molar mass $\approx$ 71.1 g/mol. %K $\approx$ (39.1 / 71.1) $\times$ 100 $\approx$ 55.0%. Option (c) KO₂ matches the given condition of containing about 55% K.

2. Paramagnetic Nature:

   • K₂O contains K⁺ and O²⁻ ions, both diamagnetic.
   • K₂O₂ contains K⁺ and O₂²⁻ (peroxide ion). The O₂²⁻ ion has all electrons paired, making it diamagnetic.
   • KO₂ contains K⁺ and O₂⁻ (superoxide ion). The superoxide ion (O₂⁻) has 13 valence electrons. According to molecular orbital theory, it has one unpaired electron in the $\pi^*$ antibonding orbital, making KO₂ paramagnetic.
   • K₂SO₄ contains K⁺ and SO₄²⁻ ions, both diamagnetic. Option (c) KO₂ is paramagnetic.

Both conditions are met by KO₂. The reaction with sulfur, followed by addition of BaCl₂, producing a white precipitate, is consistent with KO₂ forming potassium sulfite (K₂SO₃) or potassium thiosulfate (K₂S₂O₃), which then precipitate as BaSO₃ or BaS₂O₃, respectively, upon addition of BaCl₂.