Molar MassMolar mass of calcium carbonate (CaCO3)
The molar mass of calcium carbonate, CaCO3, is 100.09 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 100.09 grams.
100.09 g/mol
40.078 + 12.011 + 3 × 15.999 = 100.086 · 5 atoms in the formula · also called limestone, chalk, marble
About calcium carbonate
Calcium carbonate is limestone, chalk and marble, and also the main part of eggshells and seashells. Heating it gives quicklime and carbon dioxide. Its molar mass comes out very close to 100, which makes it a handy compound for percentage sums.
Other names: limestone, chalk, marble.
The working
The formula CaCO3 has 1 calcium atom, 1 carbon atom and 3 oxygen 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 | 40.04% |
| Carbon (C) | 1 | 12.011 | 12.011 | 12.00% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 47.96% |
40.078 + 12.011 + 3 × 15.999 = 100.086 g/mol
With the rounded atomic masses used in many school books (Ca = 40, C = 12, O = 16) the answer is 100 g/mol. The difference from 100.09 is too small to matter in most school sums.
Percentage composition
By mass, CaCO3 is 40.04% calcium, 12.00% carbon and 47.96% oxygen. The largest share comes from oxygen. In a 100 g sample there are 40.0 g of calcium, 12.0 g of carbon and 48.0 g of oxygen.
Worked examples
Grams to moles. How many moles are in 20 g of CaCO3? Divide the mass by the molar mass: 20 ÷ 100.09 = 0.1998 mol.
Moles to grams. What is the mass of 0.5 mol of CaCO3? Multiply: 0.5 × 100.09 = 50.04 g.
Counting particles. 20 g contains 0.1998 mol × 6.022 × 1023 = 1.203 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 10.01 g |
| 0.5 mol | 50.04 g |
| 1 mol | 100.1 g |
| 2 mol | 200.2 g |
| 5 mol | 500.4 g |
| Mass | Moles |
|---|---|
| 1 g | 0.009991 mol |
| 10 g | 0.09991 mol |
| 100 g | 0.9991 mol |
| 1 kg | 9.991 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CaCO3?
The molar mass of calcium carbonate (CaCO3) is 100.09 g/mol, or 100.086 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CaCO3?
100 ÷ 100.09 = 0.9991 mol.
What is the mass of one formula unit of CaCO3?
Divide the molar mass by the Avogadro constant: 100.09 ÷ 6.022 × 1023 = 1.662 × 10-22 g.
What is the percentage of oxygen in CaCO3?
Oxygen makes up 47.96% of the mass of CaCO3: 3 × 15.999 ÷ 100.086 × 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.