A white salt X gives a lilac flame (violet through cobalt glass). It gives no gas with dilute H2SO4. With conc. H2SO4 and MnO2 it gives a yellow-green gas, and with K2Cr2O7 and conc. H2SO4 it gives red vapours. A solution of 0.745 g of X is treated with excess AgNO3. The mass of precipitate formed is (K 39, Cl 35.5, Br 80, I 127, Ag 108)

JEE Advanced levelSingle correct optionDifficulty 3 of 3qa216

A white salt X gives a lilac flame (violet through cobalt glass). It gives no gas with dilute $H X_{2} SO X_{4}$ . With conc. $H X_{2} SO X_{4}$ and $MnO X_{2}$ it gives a yellow-green gas, and with $K X_{2} Cr X_{2} O X_{7}$ and conc. $H X_{2} SO X_{4}$ it gives red vapours. A solution of 0.745 g of X is treated with excess $AgNO X_{3}$ . The mass of precipitate formed is (K 39, Cl 35.5, Br 80, I 127, Ag 108)

Lilac flame means potassium; the yellow-green gas (

Cl X_{2}

) and red

CrO X_{2} Cl X_{2}

vapours identify chloride, so X is

KCl

(74.5). Moles

= 0.745/74.5 = 0.0100

and

AgCl

(143.5) forms 1:1, so mass

= 0.0100 \times 143.5 = 1.435

g. Bromide or iodide would give different masses and different gas colours.