The United States Public Health Service (USPHS) recom-mends the fluoridation of water as a means for prevent-ing

Question:

The United States Public Health Service (USPHS) recom-mends the fluoridation of water as a means for prevent-ing tooth decay. The recommended concentration is 1 mg F2/L. The presence of calcium ions in hard water can precipitate the added fluoride. What is the maximum molarity of calcium ions in hard water if the fluoride concentration is at the USPHS recommended level? (Ksp for CaF2 = 4.0 × 10-11.)