What is the concentration of each of these solutions expressed as ppm?
a. 0.0020 g iron(iii) ions dissolved in 500.0 g water.
b. 0.25 g calcium ions dissolved in 850.0 g water.

Ppm = mass of the solute in mg / mass of solvent in kg

a)

0.0020 * 1000 = 2 mg
500.0 / 1000 = 0.5 kg

ppm = 2 / 0.5

= 4 ppm


b)
 
0.25 * 1000 = 250 mg
850.0 / 1000 = 0.85 kg

ppm = 250 / 0.85

= 294.117 ppm 


hope this helps!