At a lead level of 15 ppb, what mass of lead is in a 300 mL glass of water of density 1.00 g/mL?

1. The ppb definition needs the mass of the solution, but only a volume is given. 2. Tap water can be taken to have roughly the density of pure water, about 1.00 g/mL. 3. So the solution mass is 300 mL x 1.00 g/mL = 300 g. 4. Rearranging the definition gives mass of solute = ppb x mass of solution divided by 10^9. 5. Substituting gives 15 x 300 g / 10^9 = 4.5 x 10^-6 g. 6. Converting to micrograms, 4.5 x 10^-6 g equals 4.5 micrograms of lead. 7. The value 45 micrograms would follow from dividing by 10^8, one power of ten short. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.4 Other Units for Solution Concentrations_