PPM stands for parts per million. It is a unit of concentration used to express very low concentrations of dissolved substances like trace amounts of chlorine, or other additives or impurities. For water samples this unit becomes essentially mg per litre of water. The sample of drinking water having chlorine concentration of 1.4 ppm possesses 1.4 g chlorine per 1000000 g water sample. Assuming the density of water to be 1 g/ml under experimental conditions, the mass of 250 ml water will be 250 g. Hence, mass in grams of chlorine present in it will be 1.4*250/1000000 = 0.00035g.