Grams to Moles Calculator
Grams to moles is one division: the mass over the molar mass. Type a formula and this calculator works out the molar mass, then gives the moles and the number of molecules. 100 g of water, H2O, is 5.551 mol. Switch the mode to convert moles to grams or a particle count to both.
Moles
- Moles 5,551 mmol.
- 5.551 mol
- Particles N = n × N_A = 5.551 mol × 6.02214076 × 10^23 per mol, the exact SI value of the Avogadro constant. Molecules for a molecular compound, formula units for a salt.
- 3.343 × 10^24
- Working Moles are grams divided by the molar mass: n = m / MW, with the mass in grams first. A mass left in milligrams makes the answer 1,000 times too large.
- n = m / MW = 100 g ÷ 18.015 g/mol = 5.551 mol
- Molar mass of H2O 2 × 1.0080 (H) + 15.999 (O) = 18.015 g/mol, from the IUPAC (CIAAW 2024) abridged standard atomic weights. Shown to 3 decimal places because the atomic weights are uncertain by about ±0.001 g/mol for this formula.
- 18.015 g/mol
| Element | Atoms | Atomic weight | Mass (g/mol) | Mass percent |
|---|---|---|---|---|
| H (hydrogen) | 2 | 1.0080 | 2.016 | 11.19% |
| O (oxygen) | 1 | 15.999 | 15.999 | 88.81% |
| Total | 3 | 18.015 | 100.00% |
- Molarity is per liter of SOLUTION. Dissolving 1 mol in 1 L of water gives slightly more or less than 1 L of solution, and therefore not quite 1 M. That is why the solute is dissolved first and then made up to volume.
- Molarity changes slightly with temperature because volume does. For work that must not, use molality (mol per kg of solvent).
- To dilute a stock solution to a lower concentration, use C1V1 = C2V2; the dilution calculator does that.
How the Molarity Calculator works
To convert grams to moles, divide the mass by the molar mass: n = m / M, with m in grams and M in g/mol. To convert moles to grams, multiply: m = n × M. The molar mass is the only number you have to look up, and it comes from the formula.
The worked example is 100 g of water. H2O has two hydrogens at 1.0080 and one oxygen at 15.999, so its molar mass is 18.015 g/mol. 100 / 18.015 = 5.551 mol. Going the other way, 2 mol of water is 2 × 18.015 = 36.03 g, and 0.5 mol of carbon dioxide (44.009 g/mol) is 22.00 g.
From moles to particles, multiply by the Avogadro constant. Since the 2019 redefinition of the SI it is exactly 6.02214076 × 10^23 per mole, so it carries no uncertainty of its own. The 5.551 mol of water is 3.343 × 10^24 molecules. For a salt such as NaCl the particles are formula units rather than molecules: 58.44 g of NaCl is 1.000 mol, or 6.022 × 10^23 formula units. Choose the particles mode to go back: 1 × 10^24 water molecules is 1.661 mol, which weighs 29.91 g.
The molar mass is summed from the IUPAC standard atomic weights in the 2024 abridged table, the same figures the molar mass calculator uses. Brackets and hydrates work, and the hydrate is where most wrong answers come from. 10 g of copper sulfate pentahydrate, CuSO4·5H2O at 249.68 g/mol, is 0.04005 mol. The same 10 g of anhydrous CuSO4 at 159.60 g/mol is 0.06266 mol. Type the formula of what you weighed.
Two other slips are common. A mass in milligrams has to become grams before dividing, or the answer is 1,000 times too large; the mass unit select does that conversion. And the answer is only as precise as the mass you started with. 100 g weighed to the nearest gram does not support four significant figures, whatever the calculator prints.
To turn the moles into a concentration, switch the mode to molarity and enter the volume of solution. Moles per liter of solution is molarity.
Molarity Calculator FAQ
How do you convert grams to moles?
Divide the mass in grams by the molar mass in g/mol. 100 g of water at 18.015 g/mol is 100 / 18.015 = 5.551 mol. The molar mass is the sum of the atomic weights in the formula.
How do you convert moles to grams?
Multiply the moles by the molar mass. 2 mol of water is 2 × 18.015 = 36.03 g, and 0.5 mol of CO2 is 0.5 × 44.009 = 22.00 g.
How many molecules are in a mole?
Exactly 6.02214076 × 10^23. That is the Avogadro constant, fixed by definition in the 2019 SI. Multiply moles by it to get molecules, atoms or formula units, and divide a particle count by it to get moles.
Do I use the hydrate or the anhydrous formula?
Use the formula of the substance you weighed. 10 g of CuSO4·5H2O is 0.04005 mol and 10 g of anhydrous CuSO4 is 0.06266 mol, because the water of crystallization adds 90.08 g/mol to the molar mass.