Moles Calculator
What the moles calculator does
The moles calculator converts a measured quantity into the amount of substance in moles. It has two modes. In Mass → Moles mode you enter a mass in grams and the molar mass of the substance in g/mol. In Particles → Moles mode you enter a number of atoms, molecules or ions and the tool divides by Avogadro’s number. The result is shown in moles with sensible rounding.
The two formulas
| Mode | Formula | Constants |
|---|---|---|
| Mass to moles | n = m / M | m in grams, M in g/mol |
| Particles to moles | n = N / NA | NA = 6.022 × 1023 mol−1 |
A mole is the SI unit for amount of substance. Since the 2019 redefinition it is fixed as exactly 6.02214076 × 1023 elementary entities, so the second formula no longer depends on the mass of carbon-12. The first formula still works because molar mass in g/mol is numerically equal to the relative atomic or molecular mass.
How to use it
- Pick the mode that matches what you measured: a mass on a balance, or a count of particles from a problem statement.
- Type the value. For mass, use grams; convert kilograms or milligrams first.
- In mass mode, enter the molar mass. Look it up with the molar mass calculator if you only have the formula.
- Press Calculate. The result is the amount in moles.
Worked examples
Water. 36.0 g of H₂O with molar mass 18.015 g/mol gives n = 36.0 / 18.015 = 2.00 mol.
Sodium chloride. 11.7 g of NaCl (58.44 g/mol) is n = 11.7 / 58.44 = 0.200 mol. That is the amount you would dissolve in 1.00 L for a 0.200 M solution; the molarity calculator completes that step.
Counting atoms. A sample contains 1.505 × 1024 iron atoms. n = 1.505 × 1024 / 6.022 × 1023 = 2.50 mol. Multiplying by iron’s molar mass of 55.85 g/mol gives a sample mass of 140 g.
Going the other way. If you need 0.0500 mol of glucose (180.16 g/mol), rearrange to m = n × M = 0.0500 × 180.16 = 9.01 g.
Molar masses of common substances
| Substance | Formula | Molar mass (g/mol) |
|---|---|---|
| Hydrogen gas | H₂ | 2.016 |
| Water | H₂O | 18.015 |
| Oxygen gas | O₂ | 31.998 |
| Carbon dioxide | CO₂ | 44.01 |
| Sodium chloride | NaCl | 58.44 |
| Sulfuric acid | H₂SO₄ | 98.08 |
| Calcium carbonate | CaCO₃ | 100.09 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Copper(II) sulfate pentahydrate | CuSO₄·5H₂O | 249.69 |
Values are rounded to two decimal places using IUPAC standard atomic weights. For elements with wide isotopic variation, such as lithium or boron, the fourth significant figure can differ between sources.
Finding the molar mass yourself
Add the atomic masses of every atom in the formula. For calcium carbonate: one Ca (40.08) + one C (12.01) + three O (3 × 16.00 = 48.00) = 100.09 g/mol. Count atoms inside brackets carefully: Ca(OH)₂ contains two O and two H, giving 40.08 + 2 × 16.00 + 2 × 1.008 = 74.10 g/mol. For hydrated salts, add the water molecules too.
Common mistakes
- Using atomic mass for a diatomic gas. Oxygen gas is O₂, so its molar mass is 32.00, not 16.00 g/mol.
- Entering kilograms. 0.5 kg is 500 g. The calculator expects grams.
- Confusing moles of molecules with moles of atoms. One mole of H₂O contains one mole of molecules but three moles of atoms.
- Dropping the water of crystallisation. Weighing a hydrate but using the anhydrous molar mass overstates the moles.
- Too many significant figures. Match the result to the least precise input.
Where moles fit in a calculation chain
Moles are the bridge between what you weigh and how molecules react. Once you have moles, the stoichiometry calculator applies the balanced equation, the theoretical yield calculator predicts product mass, and the percent yield calculator compares it with what you obtained. For solutions, the molarity and dilution calculators take moles and volume as inputs, and the mass percent calculator reports composition by weight.
External references
Frequently asked questions
What is the difference between molar mass and molecular weight?
Numerically they are the same. Molecular weight is a dimensionless relative mass; molar mass is that number expressed in grams per mole.
Can I enter the value in scientific notation?
Yes. Type 1.5e24 for 1.5 × 1024. This is the usual way to enter particle counts.
How many particles are in one mole?
Exactly 6.02214076 × 1023. The calculator uses 6.022 × 1023, which is accurate to four significant figures.
Does the calculator work for ions and formula units?
Yes. A mole of NaCl means a mole of formula units, each containing one Na⁺ and one Cl⁻. Use the molar mass of the whole formula.
How do I convert moles back to grams?
Multiply moles by molar mass. 0.25 mol of CO₂ is 0.25 × 44.01 = 11.0 g.