The sample of drinking water has a chlorine concentration of 1.4 parts per million (ppm).
If a substance is dissolved in water to create a solution that has a concentration of 1 ppm, there is one millionth of a gram of the substance in one gram of the sample. As the concentration of chlorine is very low, the density of the sample can be approximated to 1 g/ml. The mass of 250 ml is 250 g. This contains 250*1.4*10^-6 = 350*10^-6 g of chlorine. The mass of chlorine ions in 250 ml of the sample is 350*10^-6 g.