Case 1
- Mass of the sample
- 18g
- Molar mass
- 18.015g/mol
0.999167mol
Open with these valuesMoles Calculator
0.999167mol
Result: 0.999167 molDivide the mass by the molar mass: n = m ÷ M, grams over grams per mole leaves moles. 18 g of water at 18.015 g/mol is 0.999167 mol — almost exactly one mole, which is where the round number 18 for water comes from. Convert kilograms or milligrams to grams first.
Worked examples
Case 2
- Mass of the sample
- 58.44g
- Molar mass
- 58.44g/mol
1.000000mol
Open with these valuesCase 3
- Mass of the sample
- 100g
- Molar mass
- 18.015g/mol
5.550930mol
Open with these valuesHow it's calculated
n = m ÷ M
- StepEnter the mass of your sample in grams.
- StepEnter the molar mass in grams per mole, added up from the formula.
- ResultRead the amount of substance in moles.
What this number means
Grams and grams per mole, nothing else
The grams only cancel if the mass is in grams and the molar mass in g/mol. Kilograms or milligrams give a result off by a factor of 1000 with nothing looking wrong.
You bring the molar mass yourself
There is no substance list here; the second field takes the molar mass added up from the formula. Water is 2 × 1.008 + 15.999 ≈ 18.015 g/mol, table salt about 58.44 g/mol.
Where the round 18 for water comes from
18 g of water at 18.015 g/mol is 0.999167 mol, almost exactly one mole. At the rounded 18 g/mol, 9 g is exactly half a mole.
From moles to a number of molecules
Multiply the result by the Avogadro constant, 6.02214076 × 10²³ per mole. So 0.5 mol holds about 3.011 × 10²³ molecules.
Commonly misread
n = m × M — mass times molar mass.
It is a division, n = m ÷ M. 9 g at 18 g/mol is 0.5 mol, where multiplying would give 162.
My sample is 0.1 kg, so I enter 0.1.
Convert to grams first: 0.1 kg is 100 g, which at 18.015 g/mol gives 5.550930 mol.
18 g of water is exactly one mole.
It is 0.999167 mol. One mole of water weighs 18.015 g; the round 18 is a convenience.
Reference table
| Mass (g), molar mass (g/mol) | Substance | Amount (mol) |
|---|---|---|
| 9, 18 | Water, rounded molar mass | 0.500000 |
| 18, 18.015 | Water | 0.999167 |
| 44, 44.01 | Carbon dioxide | 0.999773 |
| 58.44, 58.44 | Sodium chloride | 1.000000 |
| 100, 18.015 | Water | 5.550930 |
Questions
How do I calculate the number of moles?
Divide the mass of your sample by the molar mass of the substance: n = m ÷ M. Use grams for the mass and grams per mole for the molar mass. For example, 18 g of water at 18.015 g/mol is 0.999167 mol.
What is a mole in chemistry?
A mole is the SI unit for the amount of substance, and one mole contains exactly 6.02214076 × 10²³ entities — atoms, molecules or formula units. That figure is the Avogadro constant, fixed by definition since 2019.
What is molar mass and how do I find it?
Molar mass is the mass of one mole in grams per mole. Add up the standard atomic weights of every atom in the formula: water is 2 × 1.008 + 15.999 ≈ 18.015 g/mol, table salt about 58.44 g/mol.
How do I convert moles to a number of molecules?
Multiply the amount of substance by the Avogadro constant: molecules = n × 6.022 × 10²³. So 0.5 mol holds about 3.011 × 10²³ molecules.
Which units must I use?
Grams for the mass and grams per mole for the molar mass, which leaves the amount in moles because the grams cancel. A mass in kilograms or milligrams has to be converted to grams first, or the result is off by a factor of 1000.
Sources and last check
Information, not professional advice.