Equivalent Weight of Hydrated Lime
| Value | 0.037046 kg/mol |
| Status | Measured: ± 0.000003 kg/mol (0.000081 relative) |
| Source | IUPAC |
| Categories | Chemistryequivalent-weightwater-treatment |
| gram per mole | 37.046 g/mol |
| kilogram per kilomole | 37.046 kg/kmol |
| milligram per millimole | 37.046 mg/mmol |
| pound per pound-mole | 37.046 lb/lb-mol |
| dalton (as molar mass) | 37.046 Da |
| kilogram per mole | 0.037046 kg/mol |
| kilodalton (as molar mass) | 0.037046 kDa |
Learning zone
Hydrated lime — slaked lime, Ca(OH)₂ — carries two hydroxides per formula unit, so its equivalent weight is half the formula mass: 74.092 / 2 = 37.046 g/eq. That is the lightest equivalent of any common alkali, lighter even than caustic soda's 40.00, and since lime is also the cheapest alkali per tonne it is the cheapest per equivalent by a wide margin — the reason municipal plants moving tonnes of alkalinity a day almost all move it as lime. Each mg/L of pure Ca(OH)₂ adds 50.04/37.05 = 1.35 mg/L of alkalinity as CaCO₃.
The catch is that lime barely dissolves: about 1.5 g/L at 20 °C, and falling as the water warms. It is therefore fed as a slurry from a slaker, with all the grit, scaling and maintenance that implies, and small systems often decide caustic's drum price buys freedom from the slurry loop. The other distinction is the cation: lime brings calcium, which in lime softening is the point — raise pH past 10.3, convert bicarbonate to carbonate, and the added calcium precipitates out with the hardness it was sent to remove. Quicklime, CaO at 56.08 g/mol, is the same alkali before slaking: 28.04 g/eq, cheaper still per equivalent, and hot enough when slaked to boil the water doing it.