Thermal decomposition of Group 2 nitrates produces NO₂ and O₂ gases in fixed ratio to identify molar mass

Steps:

• Decomposition: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂; gases mass 4(46) + 32 = 216 g per 2 moles nitrate
• Per mole M(NO₃)₂, gas mass = 108 g
• For 3.00 g sample, moles = 3 / MM; gas mass = (3 / MM) × 108 = 1.53 g
• MM = 324 / 1.53 ≈ 212 g/mol

Why D is correct:

• Sr(NO₃)₂ molar mass = 87 + 2(62) = 211 g/mol matches calculated value exactly

Why the others are wrong:

• A: Be(NO₃)₂ (133 g/mol) yields ~2.44 g gas, exceeding 1.53 g
• B: Ca(NO₃)₂ (164 g/mol) yields ~1.98 g gas, exceeding 1.53 g
• C: Mg(NO₃)₂ (148 g/mol) yields ~2.19 g gas, exceeding 1.53 g

Final answer: D