Question

The amount of lead in drinking water is 0.025 mg/L. Express this concentration in molality. {molar mass of lead is 207.2 g/mol and the density of water is 1.00 g/cm.}

Answer 1

0.025 mg = 25E-3 g Pb.
mols Pb = grams Pb/atomic mass Pb.

1000 mL H2O x 1.00 g/mL = 1000 g = 1 kg.
molality = mols Pb/kg H2O

Answer 2

1.2E-4 mols/kg