Molar MassMolar mass of calcium hydroxide (Ca(OH)2)
The molar mass of calcium hydroxide, Ca(OH)2, is 74.09 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 74.09 grams.
74.09 g/mol
40.078 + 2 × 15.999 + 2 × 1.0080 = 74.092 · 5 atoms in the formula · also called slaked lime
About calcium hydroxide
Calcium hydroxide is made by adding water to quicklime. Its clear solution is limewater, which turns milky with carbon dioxide, and its paste is whitewash. The bracket in the formula means two hydroxide groups, so there are two oxygen and two hydrogen atoms.
Other names: slaked lime.
The working
The formula Ca(OH)2 has 1 calcium atom, 2 oxygen atoms and 2 hydrogen atoms, 5 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Calcium (Ca) | 1 | 40.078 | 40.078 | 54.09% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 43.19% |
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 2.72% |
40.078 + 2 × 15.999 + 2 × 1.0080 = 74.092 g/mol
With the rounded atomic masses used in many school books (Ca = 40, O = 16, H = 1) the answer is 74 g/mol. The difference from 74.09 is too small to matter in most school sums.
Percentage composition
By mass, Ca(OH)2 is 54.09% calcium, 43.19% oxygen and 2.72% hydrogen. The largest share comes from calcium. In a 100 g sample there are 54.1 g of calcium, 43.2 g of oxygen and 2.7 g of hydrogen.
Notice that hydrogen has more atoms in the formula (2) than calcium (1), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 20 g of Ca(OH)2? Divide the mass by the molar mass: 20 ÷ 74.09 = 0.2699 mol.
Moles to grams. What is the mass of 0.5 mol of Ca(OH)2? Multiply: 0.5 × 74.09 = 37.05 g.
Counting particles. 20 g contains 0.2699 mol × 6.022 × 1023 = 1.626 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 74.09 = 7.409 g of Ca(OH)2 in each litre. For 250 mL, weigh 1.852 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 7.409 g |
| 0.5 mol | 37.05 g |
| 1 mol | 74.09 g |
| 2 mol | 148.2 g |
| 5 mol | 370.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0135 mol |
| 10 g | 0.135 mol |
| 100 g | 1.35 mol |
| 1 kg | 13.5 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Ca(OH)2?
The molar mass of calcium hydroxide (Ca(OH)2) is 74.09 g/mol, or 74.092 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Ca(OH)2?
100 ÷ 74.09 = 1.35 mol.
What is the mass of one formula unit of Ca(OH)2?
Divide the molar mass by the Avogadro constant: 74.09 ÷ 6.022 × 1023 = 1.230 × 10-22 g.
What is the percentage of calcium in Ca(OH)2?
Calcium makes up 54.09% of the mass of Ca(OH)2: 1 × 40.078 ÷ 74.092 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.