1. A 500 gm of toothpaste sample has 0.2 g of fluoride concentration. What is the

concentration of fluoride in terms of ppm level?

Here mass of solute (i.e. concentration of  F^- ion) = 0.2 gm in 500gm toothpaste

so concentration per Kg = 0.2/500 x 1000 gm = 0.4 gm/Kg 

Since to find the level of F^- ion in ppm, we need to convert the concentration in milligrams as;

 concentration of F^- ion in mg/Kg = 0.4 x 1000mg /gm = 400 mg/Kg

Hence, level of F^- ion in part per million ppm. = weight of solute / weight of solution (in kg)

so, Level of F^- ion in ppm = 400 mg/Kg = 400 ppm.