MCQ Detailed View

To identify an unknown gas, we are given its mass (24 grams) and volume (8.4 dm³) at standard temperature and pressure (STP). At STP, one mole of any gas occupies 22.4 dm³. Using this, we can calculate the molar mass of the unknown gas.

We apply the formula:
Molar mass = (Mass / Volume) × 22.4
= (24 g / 8.4 dm³) × 22.4 dm³/mol
≈ 2.857 × 22.4 ≈ 64 g/mol

Now, we compare this value with the molar masses of the options:

• 
  

  SO₃ (Sulfur trioxide) → 32 (S) + 16×3 = 80 g/mol

  
  


• 
  

  SO₂ (Sulfur dioxide) → 32 (S) + 16×2 = 64 g/mol

  
  


• 
  

  C₆H₆ (Benzene) → 12×6 + 1×6 = 78 g/mol

  
  


• 
  

  C₃H₈ (Propane) → 12×3 + 1×8 = 44 g/mol

  
  

The gas with molar mass closest to 64 g/mol is SO₂.

Therefore, the unknown gas is sulfur dioxide (SO₂).

This question belongs to Chemistry and involves understanding how gases behave at STP, how molar mass relates to volume, and how to use simple formulas to identify a gas. These kinds of problems are based on fundamental gas laws and ideal gas behavior, which are essential in chemical analysis, laboratory work, and industrial gas usage.