Convert grams to moles, moles to grams, or moles to atoms and molecules. Pick what you want to find, enter what you know, and the working is shown underneath.
What a mole actually is
A mole is a counting unit, the same idea as a dozen, only far bigger. One mole of anything is 6.022 × 1023 of it: atoms, molecules, ions, or formula units. That number is Avogadro’s number, and since 2019 it has been fixed exactly at 6.02214076 × 1023.
Chemists count in moles because atoms are too small to count one by one, but a mole of them is a mass you can put on a balance. The link between the two is the molar mass: the mass of one mole of a substance, in grams per mole. For carbon it is 12.01 g/mol, so 12.01 g of carbon holds one mole of carbon atoms.
The mole formulas
n = m ÷ M
where n is the amount in moles (mol), m is the mass in grams (g), and M is the molar mass (g/mol). Counting particles uses Avogadro’s number, written NA.
| To find | Use |
|---|---|
| Moles from mass | n = m ÷ M |
| Mass from moles | m = n × M |
| Molar mass | M = m ÷ n |
| Particles from moles | N = n × 6.022 × 1023 |
| Moles from particles | n = N ÷ 6.022 × 1023 |
Grams and particles never convert into each other directly. Moles sit in the middle, so every conversion goes through them: grams to moles to particles, or particles to moles to grams.
Finding the molar mass
Add up the atomic masses from the periodic table, once for every atom in the formula. Water, H2O, has two hydrogens and one oxygen:
2 × 1.008 + 16.00 = 18.02 g/mol
| Substance | Formula | Molar mass |
|---|---|---|
| Water | H2O | 18.02 g/mol |
| Carbon dioxide | CO2 | 44.01 g/mol |
| Sodium chloride | NaCl | 58.44 g/mol |
| Calcium carbonate | CaCO3 | 100.09 g/mol |
| Glucose | C6H12O6 | 180.16 g/mol |
For anything longer, the molar mass calculator reads a formula, brackets and hydrates included, and does the adding for you.
Worked example: grams to moles
How many moles are in 750.0 g of nickel?
Step 1. Look up the molar mass. Nickel is 58.69 g/mol.
Step 2. Divide the mass by the molar mass.
n = 750.0 g ÷ 58.69 g/mol = 12.78 mol
The grams cancel and moles are left, which is a quick check that the setup is right.
Worked example: moles to grams
A reaction needs 0.250 mol of calcium carbonate. How much do you weigh out?
Step 1. Find the molar mass of CaCO3: 40.08 + 12.01 + 3 × 16.00 = 100.09 g/mol.
Step 2. Multiply.
m = 0.250 mol × 100.09 g/mol = 25.0 g
Worked example: grams to molecules
How many water molecules are in 1.00 g of water?
Step 1. Grams to moles.
n = 1.00 g ÷ 18.02 g/mol = 0.0555 mol
Step 2. Moles to molecules.
N = 0.0555 mol × 6.022 × 1023 = 3.34 × 1022 molecules
A single gram of water is about a fifth of a teaspoon, and it holds more molecules than the commonly quoted estimate for grains of sand on every beach on Earth.
Worked example: identifying a substance
A 4.00 g sample of a metal is found to contain 0.0998 mol. What metal is it?
M = 4.00 g ÷ 0.0998 mol = 40.1 g/mol, which matches calcium at 40.08 g/mol. Working out a molar mass this way is a standard method for identifying an unknown element or compound.
Moles of molecules and moles of atoms
One mole of carbon dioxide is one mole of CO2 molecules, but each molecule holds three atoms. So the same sample holds one mole of carbon atoms, two moles of oxygen atoms, and three moles of atoms in total.
Read the question carefully for which one it wants. In the water example above, 3.34 × 1022 molecules contain 6.68 × 1022 hydrogen atoms. For ionic compounds such as NaCl, the counted unit is the formula unit rather than a molecule, but the arithmetic is identical.
From moles of one substance to another
Chemical equations are written in moles, so the coefficients give the mole ratio between substances. In 2H2 + O2 → 2H2O, two moles of hydrogen make two moles of water. To go from grams of one reactant to grams of a product, convert to moles, apply the ratio, then convert back to grams. The stoichiometry calculator runs the whole chain from a balanced equation.
Mistakes to avoid
- Dividing the wrong way. Moles are mass divided by molar mass. If the units come out as 1/mol instead of mol, the fraction is upside down.
- Using the atomic mass of one element for a compound. Oxygen gas is O2, so its molar mass is 32.00 g/mol, not 16.00.
- Forgetting the subscripts. In CaCO3 the oxygen counts three times.
- Skipping moles when counting particles. Grams times Avogadro’s number is not a particle count.
- Mixing up molecules and atoms. Decide which the question asks for before the last step.
- Rounding too early. Keep extra digits through the working and round once at the end.
Frequently asked questions
What is the mole formula?
The number of moles equals the mass of a substance divided by its molar mass: n = m ÷ M. Rearranged, mass is moles times molar mass, and molar mass is mass divided by moles.
How do I convert grams to moles?
Divide the mass in grams by the molar mass in grams per mole. For example, 36.04 g of water divided by 18.02 g/mol is 2.000 mol.
How do I convert moles to grams?
Multiply the number of moles by the molar mass. For example, 0.250 mol of calcium carbonate times 100.09 g/mol is 25.0 g.
How do I find the molar mass?
Add the atomic masses from the periodic table for every atom in the formula. Water, H2O, is 2 × 1.008 + 16.00, which is 18.02 g/mol.
How many particles are in a mole?
One mole contains Avogadro’s number of particles, 6.02214076 × 10^23, usually rounded to 6.022 × 10^23. The particles can be atoms, molecules, ions, or formula units.
How do I convert molecules to moles?
Divide the number of molecules by 6.022 × 10^23. For example, 5.00 × 10^24 molecules of carbon dioxide is 8.30 mol.
Related terms
Next: turn moles into a concentration with the molarity calculator, or read the mole and Avogadro’s number for the history behind the unit.