The daily intake of iron is 18 mg for an adult. This dose dissolved as Fe2+ ions in 1 liter of water makes an 18 ppm Fe2 solution. Convert 18 ppm Fe2 to molarity. Assume the aqueous solution is dilute enough to use the shortcut and the solution density is essentially equal to water (1.0 g/mL). Answer to 2 significant figures with appropriate units.