An aqueous solution contains 0.10 MHS and 0.20 M HCl. If the equilibrium constants for the formation of HS- from HS is 1.0 x 10-7 and that of S2- from HS-ions is 1.2 x 10-13 then the concentration of S2- ions in aqueous solution is : (2) 3 10-20 (4) 5 10-19 (1) 510-8 (3) 6 10-21