Identify metal by atomic mass from gas yield in nitrate decomposition

Steps:

• Decomposition: 2M(NO3)₂ → 2MO + 4NO₂ + O₂; 216 g gas (4×46 + 32) from 2 mol salt, so 108 g gas/mol salt.
• Moles of salt = gas mass / 108 = 1.32 / 108 ≈ 0.01222 mol.
• Molar mass of salt = 2.00 g / 0.01222 mol ≈ 163.6 g/mol.
• Atomic mass of M = 163.6 - 124 (2×62 for NO₃) ≈ 39.6 g/mol ≈ 40 for Ca.

Why B is correct:

• Calcium atomic mass (40) matches calculated value from gas mass ratio.

Why the others are wrong:

• A: Beryllium (9) gives salt molar mass 133; moles higher, gas >1.32 g.
• C: Magnesium (24) gives salt molar mass 148; moles higher, gas >1.32 g.
• D: Strontium (88) gives salt molar mass 212; moles lower, gas <1.32 g.

Final answer: B