Atomic Mass of Calcium
| Value | 0.040078 kg/mol |
| Status | Measured: ± 0.000004 kg/mol (0.0001 relative) |
| Source | IUPAC |
| Categories | Chemistryatomic-masswater-treatment |
| gram per mole | 40.078 g/mol |
| kilogram per mole | 0.040078 kg/mol |
Learning zone
Calcium at 40.078 g/mol and a charge of 2+ has an equivalent weight of 20.04 g/eq, which is what makes the hardness conversion work: multiply mg/L of Ca²⁺ by 100.09/40.08 = 2.497 and you have mg/L as CaCO₃. A water with 60 mg/L of calcium therefore contributes 150 mg/L of hardness, and hardness is simply the sum of calcium and magnesium expressed on that common scale.
It is the fifth most abundant element in the crust and the most abundant metal in the human body, essentially all of it locked in bone as hydroxyapatite. In a heating system it is the villain: calcium carbonate's inverse solubility means it comes out of solution on the hottest surface available, and a millimetre of scale on a boiler tube can cost several percent of fuel efficiency. Humphry Davy isolated the metal in 1808, again by electrolysis.