The amount of good quality bleaching powder required for chlorinating a round well measuring 4 mtrs diameter and depth of water column 6 mts, to get PPM of residual chlorine when the Horrok&;s test gives definite blue colour in the 3rd cup onwards, is -

Simple solution: The 3rd cup becomes blue in Horrocks apparatus = 3x2 = 6 gm of bleaching powder required to disinfect 455 L of water. That means 6/455 gm/L of Bleaching powder is required The volume of well (V) = p r2 h, here r = d/2 = 2m & h = 6m V = 3.14 x 4 x 6 = 75.36 m3 = 75.36 x 1000 L = 75360 L Total Bleaching powder required for well = 75360 x 6/455 = 993.75