Molar mass of the salt from NaBr, NaNO₃, KI and CaF₂ which does not evolve coloured vapours on heating with concentrated H₂ SO₄ is ________ g mol ⁻¹.
(Molar mass in g mol⁻¹: Na: 23, N: 14, K: 39, O: 16, Br: 80, I: 127, F: 19, Ca: 40)

Solution

Chemistry ma’am · 3 steps

Three of these anions give you a coloured gas with hot concentrated acid; fluoride gives only colourless HF.

1. What the acid does. Hot concentrated H₂SO₄ first forms the volatile acid of each anion. It is also an oxidising agent, so it can push HBr, HI and HNO₃ further, into coloured gases.

   Salt	What comes off	Colour of vapour
   NaBr	HBr is oxidised to Br₂	reddish brown
   NaNO₃	HNO₃ decomposes to NO₂	brown
   KI	HI is oxidised to I₂	violet
   CaF₂	HF only	colourless

2. Why fluoride differs. CaF₂ + H₂SO₄ → CaSO₄ + 2HF. Concentrated H₂SO₄ is far too weak an oxidant to turn F⁻ into F₂, so only colourless HF comes off.

   M(CaF₂) = 40 + 2 × 19 = 78 g mol⁻¹

3. So. The salt is CaF₂, and its molar mass is 78 g mol⁻¹.

The common slip: 59. Counting only one fluorine gives 40 + 19 = 59. Calcium is +2, so the formula has two fluorines and the molar mass is 78.

The idea to keep. With hot concentrated H₂SO₄, bromide, iodide and nitrate all give coloured gases. Fluoride and chloride give only colourless HF or HCl, because the acid cannot oxidise them.

Answer from the NTA final answer key. Solution by PrepWiser.