Molar MassMolar mass of calcium oxide (CaO)
The molar mass of calcium oxide, CaO, is 56.08 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 56.08 grams.
56.08 g/mol
40.078 + 15.999 = 56.077 · 2 atoms in the formula · also called quicklime
About calcium oxide
Calcium oxide is quicklime, made by heating limestone. It reacts with water, giving out much heat, to form slaked lime. It is used in making cement and steel.
Other names: quicklime.
The working
The formula CaO has 1 calcium atom and 1 oxygen atom, 2 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 | 71.47% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 28.53% |
40.078 + 15.999 = 56.077 g/mol
With the rounded atomic masses used in many school books (Ca = 40, O = 16) the answer is 56 g/mol. The difference from 56.08 is too small to matter in most school sums.
Percentage composition
By mass, CaO is 71.47% calcium and 28.53% oxygen. The largest share comes from calcium. In a 100 g sample there are 71.5 g of calcium and 28.5 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of CaO? Divide the mass by the molar mass: 25 ÷ 56.08 = 0.4458 mol.
Moles to grams. What is the mass of 0.1 mol of CaO? Multiply: 0.1 × 56.08 = 5.608 g.
Counting particles. 25 g contains 0.4458 mol × 6.022 × 1023 = 2.685 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 5.608 g |
| 0.5 mol | 28.04 g |
| 1 mol | 56.08 g |
| 2 mol | 112.2 g |
| 5 mol | 280.4 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01783 mol |
| 10 g | 0.1783 mol |
| 100 g | 1.783 mol |
| 1 kg | 17.83 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CaO?
The molar mass of calcium oxide (CaO) is 56.08 g/mol, or 56.077 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CaO?
100 ÷ 56.08 = 1.783 mol.
What is the mass of one formula unit of CaO?
Divide the molar mass by the Avogadro constant: 56.08 ÷ 6.022 × 1023 = 9.312 × 10-23 g.
What is the percentage of calcium in CaO?
Calcium makes up 71.47% of the mass of CaO: 1 × 40.078 ÷ 56.077 × 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.