Question

Federal law requires that drinking water contain no more than 2.49 x 10-6 M of mercury. Assuming that mercury is present as Hg2+, what is this Hg2+ concentration expressed in PARTS PER MILLION? (Molar Mass of Hg = 201 g/mol)

Answer 1

Let volume be 1 L
since density of water = 1 Kg/L
mass of water = 1 Kg = 1000 g

M = number of moles / volume of solution in L
2.49*10^-6 = n / 1
n = 2.49*10^-6 mol

Molar mass of Hg= 201 g/mol
mass of Hg = number of moles * molar mass
                       = (2.49*10^-6)*201
                       = 5.005*10^-4 g

ppm = mass of Hg * 10^6 / total mass
          = (5.005*10^-4)*10^6 / (5.005*10^-4 + 1000)
          = 0.5
Answer: 0.5