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A hard-water sample contains Ca(HCO3)2 at a concentration of 405 ppm. The density of the water is 1.0 g/mL. If the molarity of the solution is x * 10⁻³ M, find x. (Molar mass of Ca(HCO3)2 = 162 g/mol.)

1. 2.5
2. 5.0
3. 1.25
4. 4.05

Correct answer: 2.5

Solution

405 ppm = 405 mg/L = 0.405 g/L; dividing by 162 g/mol gives 2.5 * 10⁻³ M, so x = 2.5.

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