Q. Molar mass of the salt from NaBr, NaNO₃, KI and CaF₂ which does not
evolve coloured vapours on heating with concentrated H₂SO₄ is ______ g mol⁻¹.
( Molar mass in g mol⁻¹ : Na : 23, N : 14, K : 39, O : 16,
Br : 80, I : 127, Ca : 40, F : 19 )
Explanation
Step 1: Behaviour with concentrated H₂SO₄
NaBr and KI evolve coloured vapours (Br₂ – brown, I₂ – violet).
Hence, both are eliminated.
Step 2: Check remaining salts
NaNO₃ gives nitric acid vapours (colourless).
CaF₂ gives HF gas (colourless).
Step 3: Salt that does not evolve coloured vapours
NaNO₃ is selected.
Step 4: Calculate molar mass of NaNO₃
Na = 23
N = 14
O₃ = 16 × 3 = 48
Total molar mass = 23 + 14 + 48 = 85 g mol⁻¹
Correct Answer = 78