CaCO₃ Molar Mass
Calcium carbonate
The molar mass of CaCO₃ (calcium carbonate) is approximately 100.09 g/mol. Each formula unit combines one calcium atom, one carbon atom, and three oxygen atoms, so the sum using the listed atomic weights is 40.078 + 12.011 + (3 × 15.999) = 100.086 g/mol — about 40.0% calcium, 12.0% carbon, and 48.0% oxygen by mass. Because CaCO₃ is an ionic compound, this same number is properly called the formula mass.
Molar mass of CaCO₃
100.09 g/mol
Atomic-weight sum before rounding: 100.086 g/mol
| Formula | CaCO₃ |
|---|---|
| Molar Mass | 100.09 g/mol |
| Formula Mass | ≈ 100.086 u |
| Calcium Atoms | 1 |
| Carbon Atoms | 1 |
| Oxygen Atoms | 3 |
| Atoms per Formula Unit | 5 |
Good to know
CaCO₃ is an ionic compound: a crystal lattice of Ca²⁺ and CO₃²⁻ ions with no discrete molecules. That is why chemists quote a formula mass per formula unit rather than a molecular mass.
How to Calculate the Molar Mass of CaCO₃
The calcium carbonate molar mass is the sum of one calcium atomic weight, one carbon atomic weight, and three oxygen atomic weights. Using the listed, rounded standard atomic weights gives approximately 100.086 g/mol — rounded to 100.09 g/mol for classroom use. The same number also goes by "formula mass of calcium carbonate", because CaCO₃ is an ionic compound counted in formula units rather than discrete molecules.
CaCO₃
- Calcium: 1 × 40.078 = 40.078
- Carbon: 1 × 12.011 = 12.011
- Oxygen: 3 × 15.999 = 47.997
40.078 + 12.011 + 47.997 = 100.086 g/mol
≈ 100.09 g/mol
Step 1 Read the chemical formula
CaCO₃ contains 1 calcium atom, 1 carbon atom and 3 oxygen atoms. Subscripts count atoms, and a symbol without a subscript counts one.
Step 2 Find the atomic masses
Use the listed, rounded standard atomic weights: Ca = 40.078, C = 12.011 and O = 15.999. Most atomic weights are not whole numbers — each one averages an element's naturally occurring isotopes.
Step 3 Multiply by the atom counts
Each element contributes its atomic weight once per atom: Ca: 1 × 40.078 = 40.078; C: 1 × 12.011 = 12.011; O: 3 × 15.999 = 47.997.
Step 4 Add the contributions
40.078 + 12.011 + 47.997 = 100.086 g/mol. The total using these atomic weights is approximately 100.086 g/mol, usually reported as 100.09 g/mol. One mole of CaCO₃ — 6.022 × 10²³ formula units, 5 atoms each — has approximately this mass in grams.
CaCO₃ Formula Breakdown
CaCO₃
- Ca = Calcium × 1
- C = Carbon × 1
- O = Oxygen × 3
- Total atoms = 5
CaCO₃ reads as two charged pieces, not four separate atoms glued in a row: a Ca²⁺ ion paired with a carbonate unit, CO₃²⁻. The subscript ₃ belongs to that carbonate unit — one carbon bonded to three oxygens, which travel together as a single 2− ion — so the oxygen count multiplies by 3 while Ca and C each count once. Add the pieces up: 1 + 1 + 3 = 5 atoms per formula unit.
Elemental Composition of CaCO₃
Each element's share of the total mass of one mole of CaCO₃. One mole — about 100.09 g — contains 40.078 g of calcium, 12.011 g of carbon and 47.997 g of oxygen, and every sample scales by the same ratio. Stated as percent composition, CaCO₃ is 40.04% calcium, 12.00% carbon and 47.96% oxygen by mass.
| Element | Atoms | Atomic Weight | Mass Contribution | 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% |
| Total | 5 | 100.086 | 100% |
What Is the Formula Mass of CaCO₃?
The formula mass of CaCO₃ is approximately 100.09 u per formula unit — numerically the same number as the molar mass, 100.09 g/mol, because one mole of formula units weighs as many grams as one formula unit weighs in unified atomic mass units. A formula unit here is one Ca²⁺ ion paired with one CO₃²⁻ ion. Like solid NaCl, calcium carbonate is a crystal lattice in which the ions alternate in a fixed ratio, so there are no discrete "CaCO₃ molecules" to weigh — the formula unit is the counting unit that plays the molecule's role.
You will still see "molecular mass of CaCO₃" in search results and homework questions. It is used informally, and it always expects the number 100.09 — but the more precise terms are formula mass and formula weight, which describe exactly what CaCO₃ has: a mass per formula unit. Reference tables such as Wikipedia list 100.0869 g/mol, computed from a slightly older atomic-weight set; the values used here give 100.086, and both round to the classroom value 100.09.
| Molar mass | 100.09 g/mol — the mass of one mole of CaCO₃ formula units |
|---|---|
| Formula mass | ≈ 100.086 u (Da) — the mass of one single CaCO₃ formula unit |
What Is the Chemical Formula for Calcium Carbonate?
The chemical formula for calcium carbonate is CaCO₃. The compound is built from ions, not neutral atoms: each calcium atom loses two electrons to become a Ca²⁺ ion, and the carbonate ion CO₃²⁻ — one carbon bonded to three oxygens — carries a matching 2− charge.
The charges cancel in a 1 : 1 ratio, so one Ca²⁺ pairs with one CO₃²⁻ and the formula needs no extra subscripts: Ca²⁺ + CO₃²⁻ → CaCO₃. That 1 : 1 pairing is also why a formula unit contains exactly five atoms — one calcium plus the carbonate unit's one carbon and three oxygens.
How Many Grams Are in One Mole of CaCO₃?
One mole of CaCO₃ has a mass of approximately 100.09 grams. That is the whole conversion — the molar mass is the conversion factor between moles and grams. 1 mol × 100.09 g/mol = 100.09 g, and half a mole weighs 0.5 × 100.09 = 50.045 g. Going the other way, divide by the same factor: 200.18 g of CaCO₃ is 200.18 ÷ 100.09 = 2 mol. Type an amount below to convert either way; the calculation uses 100.09 g/mol.
1 mol of CaCO₃ has a mass of 100.09 g
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CaCO₃ Molar Mass Units and Conversions
Molar mass is normally quoted in grams per mole (g/mol), but the same number scales across units: 100.09 g/mol = 100,090 mg/mol = 0.10009 kg/mol. Per formula unit, the same value appears in unified atomic mass units — one CaCO₃ formula unit has a formula mass of about 100.09 u. The number never changes; only the unit — and the amount of CaCO₃ it describes — does.
CaCO₃ molar mass in every unit
- grams per mole: 100.09 g/mol
- milligrams per mole: 100,090 mg/mol
- kilograms per mole: 0.10009 kg/mol
- per formula unit: ≈ 100.086 u
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Frequently Asked Questions
What is the molar mass of CaCO₃?
Approximately 100.09 g/mol. Using IUPAC standard atomic weights, the sum is 1 × 40.078 (calcium) + 1 × 12.011 (carbon) + 3 × 15.999 (oxygen) = 100.086 g/mol, which rounds to 100.09 g/mol.
How do you calculate the molar mass of CaCO₃?
Add one calcium, one carbon, and three oxygen atomic-mass contributions: 40.078 + 12.011 + 47.997 = 100.086 ≈ 100.09 g/mol. In general: read the subscripts in the formula, multiply each element's atomic weight by its atom count, then add the contributions — exactly what the worked example above shows.
What is the formula mass of CaCO₃?
Approximately 100.09 u per formula unit. One formula unit is one Ca²⁺ ion paired with one CO₃²⁻ ion; numerically, formula mass in u and molar mass in g/mol are the same number.
How many atoms are in CaCO₃?
Five in total: one calcium atom, one carbon atom, and three oxygen atoms in every formula unit. The three oxygens belong to the carbonate unit, CO₃²⁻, which travels as a single 2− ion.
What percentage of CaCO₃ is calcium?
About 40% by mass — approximately 40.04% using the listed atomic weights (40.078 of 100.086). Carbon contributes 12.00%, and the three oxygens carry the remaining 47.96%, which is why nearly half of any calcium carbonate sample's mass is oxygen.
How many grams are in one mole of CaCO₃?
About 100.09 grams. One mole contains 6.022 × 10²³ formula units, and the molar mass converts that amount directly to mass: 1 mol × 100.09 g/mol = 100.09 g.
What is CaCO₃ called?
Calcium carbonate. The name comes straight from the ions: the Ca²⁺ cation is calcium and the CO₃²⁻ anion is carbonate.
Does CaCO₃ have an atomic number?
No — atomic number is a property of elements, not compounds. The elements inside CaCO₃ each have one: calcium is 20, carbon is 6, and oxygen is 8. A compound like CaCO₃ is identified by its formula, not an atomic number.
Atomic weight reference
Calculations use the rounded standard atomic weights shown in the breakdown, not exact isotope masses. Natural isotope composition can vary, and some standard atomic weights are reported as intervals. See the CIAAW standard atomic weights for the reference values. Use your course’s table when its rounding differs.