Molar Mass of Calcium Carbonate

Calcium carbonate's convenient 100.086 g/mol, and its equivalent weight of almost exactly 50 g/eq, are the reason the entire water industry reports hardness and alkalinity "as CaCO₃" rather than in the ions actually present. A water with 80 mg/L of Ca²⁺ and 12 mg/L of Mg²⁺ is converted to a single number, 249 mg/L as CaCO₃, by scaling each ion by 50 divided by its own equivalent weight. It is a unit of convenience, not a claim that any calcium carbonate is dissolved.

The same compound is limestone, chalk, marble, eggshell and the scale that furs a kettle element. Because its solubility falls as temperature rises — the opposite of most salts — it precipitates preferentially on the hottest surface in a system, which is exactly the heat-exchanger tube you least want coated. The Langelier index exists to predict whether a given water will lay that scale down or dissolve it.