A 100.0mL sample of the lake water was treated with 1.0mL of 1M AgNO3. 71.7mg of silver (I) chloride was isolated.What is the concentration of chloride ions in the water sample?

Answer 1

(Cl^-) = often expressed as mg/L which is easily converted t percent. You don't list a unit.
71.7 mg AgCl. Convert to mg Cl.
71.7 mg AgCl x (atomic mass Cl/molar mass AgCl) = about 18 mg Cl
(Cl^-) = 18 mg/100 mL (which is 0.018%) or 180 mg/L