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Concentrations in very dilute solutions are often quoted in parts per million (ppm) or parts per billion (ppb), defined as grams of solute per million or per billion grams of solvent. The Bay of Bengal contains 2.1 ppm of lithium ions. If the molality of Li+ is x * 10⁻⁴ m, find x. (Atomic mass of Li = 7)

1. 3
2. 2.1
3. 7
4. 1.5

Correct answer: 3

Solution

2.1 g Li per 1000 kg water gives moles/kg = (2.1/7)/1000 = 3 * 10⁻⁴ m, so x = 3.

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