The molar mass of calcium hydroxide (Ca(OH)2) is 74.092 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
or
.
| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Calcium (Ca) | 1 | 40.078 | 40.078 | 54.09% |
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 2.72% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 43.19% |
| Total | 5 | 74.092 | 100.00% |
40.078 + 2.016 + 31.998 = 74.092 g/mol
Grams and moles
25 g of calcium hydroxide is 0.3374 mol, because 25 ÷ 74.092 = 0.3374.
These elements have no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: hydrogen is 1.0080, and really lies between 1.00784 and 1.00811; oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Slaked lime is 74.092 g/mol: calcium 40.078, two oxygens 31.998 and two hydrogens 2.016. Calcium alone carries 54.09% of the mass.
It sits in the middle of the lime cycle, which is a neat sequence of molar masses. Heat calcium carbonate at 100.086 and carbon dioxide leaves, giving quicklime at 56.077. Add water at 18.015 and you get this compound at 74.092, since 56.077 plus 18.015 is exactly 74.092. Leave it exposed to air and it reabsorbs carbon dioxide, returning to calcium carbonate. Lime mortar sets by completing that last step, which is why old mortar hardens over decades rather than minutes.
Adding water to quicklime releases enough heat to boil the water, which is where the name slaking comes from.
Limewater, the saturated solution, is the classic test for carbon dioxide: it turns milky as insoluble calcium carbonate forms.