Argentite (Ag2S, molar mass 248) is treated as follows: Ag2S + 4NaCN -> 2Na[Ag(CN)2] + Na2S; the sulphide is oxidised by 2Na2S + 2O2 + H2O -> Na2S2O3 + 2NaOH; the silver is then precipitated with zinc, 2Na[Ag(CN)2] + Zn -> Na2[Zn(CN)4] + 2Ag. For 24.8 g of pure Ag2S treated completely (Zn = 65, O = 16), the mass of zinc consumed and the mass of O2 consumed in the sulphide oxidation step are respectively

JEE Advanced levelSingle correct optionDifficulty 3 of 3mt152

Argentite ( $Ag X_{2} S$ , molar mass $248$ ) is treated as follows: $Ag X_{2} S + 4 NaCN 2 Na [Ag (CN) X_{2}] + Na X_{2} S$ ; the sulphide is oxidised by $2 Na X_{2} S + 2 O X_{2} + H X_{2} O Na X_{2} S X_{2} O X_{3} + 2 NaOH$ ; the silver is then precipitated with zinc, $2 Na [Ag (CN) X_{2}] + Zn Na X_{2} [Zn (CN) X_{4}] + 2 Ag$ . For $24.8 g$ of pure $Ag X_{2} S$ treated completely (Zn = 65, O = 16), the mass of zinc consumed and the mass of $O X_{2}$ consumed in the sulphide oxidation step are respectively

24.8/248 = 0.100 mol

Ag X_{2} S

gives

0.200 mol

Na [Ag (CN) X_{2}]

and

0.100 mol

Na X_{2} S

. Zinc needed is

0.100 mol

(

1

Zn per

2

Ag), i.e.

6.50 g

. Oxygen needed is

1

per

Na X_{2} S

, i.e.

0.100 mol

, which is

3.20 g