The EPA maximum safe level of fluoride ion in tap water is 4 ppm. At that level, what mass of fluoride is in 1.00 L of tap water, taking its density as 1.00 g/mL?

1. With a density of 1.00 g/mL, 1.00 L of tap water has a mass of 1000 g. 2. The ppm unit is mass of solute over mass of solution times 10^6. 3. Rearranging, mass of solute = 4 x 1000 g / 10^6 = 4.0 x 10^-3 g. 4. Converting gives 4.0 mg of fluoride ion. 5. The value 0.0040 mg mistakes the answer in grams for an answer in milligrams, shrinking it a thousandfold. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.4 Other Units for Solution Concentrations_