The units of parts per million (ppm) and parts per billion (ppb) are commonly used by environmental chemists. In general, 1 ppm means 1 part of solute for every 106 parts of solution. (Both solute and solution are measured using the same units.) Mathematically, by mass:

In the case of very dilute aqueous solutions, a concentration of 1.0 ppm is equal to 1.0 mg of solute per 1.0 mL of solution, which equals 1.0 g of solution. Parts per billion is defined in a similar fashion. Calculate the molarity of each of the following aqueous solutions.

a. 5.0 ppb Hg in H₂O

b. 1.0 ppb CHCl₃ in H₂O

c. 10.0 ppm As in H₂O

d. 0.10 ppm DDT (C₁₄H₉Cl₅) in H₂O

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ppm = μg solute g solution mg solute kg solution