We check which salt does not evolve coloured vapours on heating with concentrated $\mathrm{H_2SO_4}$.

Consider each salt:

(i) $\mathrm{NaBr}$

With conc. $\mathrm{H_2SO_4}$, bromide first gives $\mathrm{HBr}$, which is then oxidized to bromine: $2\mathrm{HBr} + \mathrm{H_2SO_4} \rightarrow \mathrm{Br_2} + \mathrm{SO_2} + 2\mathrm{H_2O}$ Bromine vapours are reddish brown (coloured).

(ii) $\mathrm{KI}$

Iodide gives $\mathrm{HI}$, which is oxidized to iodine: $2\mathrm{HI} + \mathrm{H_2SO_4} \rightarrow \mathrm{I_2} + \mathrm{SO_2} + 2\mathrm{H_2O}$ (Further reduction products may also form.) Iodine vapours are violet (coloured).

(iii) $\mathrm{NaNO_3}$

On heating with conc. $\mathrm{H_2SO_4}$: $\mathrm{NaNO_3} + \mathrm{H_2SO_4} \rightarrow \mathrm{NaHSO_4} + \mathrm{HNO_3}$ Nitric acid fumes are not the typical coloured vapours asked here.

(iv) $\mathrm{CaF_2}$

On heating with conc. $\mathrm{H_2SO_4}$: $\mathrm{CaF_2} + \mathrm{H_2SO_4} \rightarrow \mathrm{CaSO_4} + 2\mathrm{HF}$ $\mathrm{HF}$ vapours are colorless.

Among the given salts, the one classically identified as not evolving coloured vapours with conc. $\mathrm{H_2SO_4}$ is $\mathrm{CaF_2}$.

Calculate its molar mass: $M(\mathrm{CaF_2}) = 40 + 2(19) = 40 + 38 = 78\ \mathrm{g\ mol^{-1}}$

Therefore, the required molar mass is: $\boxed{78}$