Q81 (2024)
Molar mass of the salt from NaBr, $NaNO_3$, KI and $CaF_2$ which does not evolve coloured vapours on heating with concentrated $H_2SO_4$ is ________ $\text{g } mol^{-1}$.
(Molar mass in $\text{g } mol^{-1}$: Na: 23, N: 14, K: 39, O: 16, Br: 80, I: 127, F: 19, Ca: 40)
### Step 1: Analyzing the reactions with Concentrated H2SO4
$NaBr \xrightarrow{\text{conc. } H_2SO_4} Br_2$ (Reddish-brown vapour evolves)
$NaNO_3 \xrightarrow{\text{conc. } H_2SO_4} NO_2$ (Brown vapour evolves)
$KI \xrightarrow{\text{conc. } H_2SO_4} I_2$ (Violet vapour evolves)
$CaF_2 \xrightarrow{\text{conc. } H_2SO_4} HF$ (Colorless vapour evolves)
Thus, $CaF_2$ does not evolve colored vapours.
### Step 2: Calculating Molar Mass
Molar mass of $CaF_2$:
$Ca = 40$
$$F_2 = 2 \times 19 = 38$$
$$\text{Molar mass} = 40 + 38 = 78 \text{ g/mol}$$
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Salt Analysis
