Units of parts per million (ppm) or parts per billion (ppb) are often used to describe the concentrations of solutes in very dilute solutions. The units are defined as the number of grams of solute per million or per billion grams of solvent. Bay of Bengal has 1.89 ppm of lithium ions. The molality of Li⁺ in this water is (atomic number of Li=7):
a. 1.5 x 10⁻⁴ m
b. 1.7 x 10⁻⁴ m
c. 2.5 x 10⁻⁴ m
d. 2.7 x 10⁻⁴ m