Calculate molar mass using ideal gas law

Steps:

• Convert volume: 400 cm³ = 0.0004 m³.
• Compute moles: n = (101 kPa × 0.0004 m³) / (8.314 J/mol·K × 300 K) = 0.0162 mol.
• Determine molar mass: M = 0.65 g / 0.0162 mol ≈ 40 g/mol.
• Identify gas: Matches argon's atomic mass of 40 g/mol.

Why A is correct:

• Argon's molar mass is 40 g/mol, exactly matching the calculated value from PV = nRT.

Why the others are wrong:

• B. Helium: Molar mass 4 g/mol, far below 40 g/mol.
• C. Krypton: Molar mass 84 g/mol, twice the calculated value.
• D. Neon: Molar mass 20 g/mol, half the required mass.

Final answer: A