The permissible available chlorine in treated water should be 0.25 mg L-1.  Bleaching powder to be added into 1000 L tank for the permissible chlorine should be

Available chlorine is due to bleaching powder.CaOCl2 +H2O →   CaOH2 +Cl2 127 g                                              71 g                                                        0.25 mg L-1                                                        0.25 g/1000 L Cl2  required in 1000 L tank = 0.25 × 1000                                                    = 250 mg                                                    = 0.25 g 71 g of Cl2 is form = 127 g CaOCl2 0.25 g of Cl2  is from = 127 × 0.2571 = 0.45 g