Moles to grams for water is a multiplication by 18.015 g/mol. A mole weighs 18.015 g, 100 mmol weighs 1.8015 g, 10 mmol weighs 0.18015 g and a millimole is 18.015 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
The figure comes from the formula. Water is 2 × 1.008 (H) + 15.999 (O), which is 18.015 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is oxygen, and they account for 15.999 g of the 18.015 g, or 88.81% by mass. The derivation table below breaks the whole molecule down element by element.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
The reference substance for almost everything. One mole is 18.015 g, which is 18.0 mL at room temperature — the only compound where those numbers coincide. At 18.015 g/mol it is heavier than 2 of the 11 gases and small molecules covered here, and that ranking matters more than it looks: ammonia has a molar mass of 17.031 g/mol, so a gram of it contains 5.78% more formula units than a gram of water. Weigh by mass and you are not weighing equal amounts of substance.
Because this is a gas, volume is often the easier measure. One mole of any ideal gas occupies 22.414 L at 0 °C and 1 atm and 24.4655 L at 25 °C, so 18.015 g of water — one mole — fills about 24.47 litres at room temperature. Real gases depart from that by a percent or so, more near their boiling point, but for ordinary work the molar volume is the quickest route from grams to litres.
The formula
n- The amount of water you need, in moles
18.015- The molar mass of water in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 18.015 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of water in grams
5 mmol × 18.015 g/mol = 0.090075 g, which is 90.075 mg. Doubling the amount doubles the mass to 0.18015 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of water in grams
250 mmol × 18.015 g/mol = 4.50375 g, which is 4503.75 mg. Doubling the amount doubles the mass to 9.0075 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of water in grams
2 mol × 18.015 g/mol = 36.03 g, which is 36,030 mg. Doubling the amount doubles the mass to 72.06 g, because the relationship is a plain proportion once the molar mass is fixed.
How to read your score
Frequently asked questions
What is the molar mass of water (H2O)?
It is 18.015 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 1.008 (H) + 15.999 (O). One mole of water therefore weighs 18.015 g, and a gram of it is 55.509 mmol.
What percentage of water is oxygen?
88.81% by mass. Each formula unit contains 1 oxygen atom contributing 15.999 g of the 18.015 g total, so 100 g of water contains 88.81 g of oxygen and a kilogram contains 888.09 g of it.
How many grams is one mole of water?
18.015 g, by definition of the molar mass. Two moles is 36.03 g and half a mole is 9.0075 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of water is 1.8015 g. One millimole is 18.015 mg and one micromole is 18.015 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 0.1802 mg of water, weigh 1.802 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, H2O, at 18.015 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams reference for water
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 11.19% |
| Total — one mole of water | 18.015 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 18.015 g/mol.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000018 | 0.018015 | 0.000018 |
| 10 µmol | 0.00018015 | 0.18015 | 0.000183753 |
| 100 µmol | 0.0018015 | 1.8015 | 0.00183753 |
| 1 mmol | 0.018015 | 18.015 | 0.0183753 |
| 10 mmol | 0.18015 | 180.15 | 0.183753 |
| 50 mmol | 0.90075 | 900.75 | 0.918765 |
| 100 mmol | 1.8015 | 1801.5 | 1.83753 |
| 250 mmol | 4.50375 | 4503.75 | 4.59382 |
| 500 mmol | 9.0075 | 9007.5 | 9.18765 |
| 750 mmol | 13.5113 | 13,511.2 | 13.7815 |
| 1 mol | 18.015 | 18,015 | 18.3753 |
| 2 mol | 36.03 | 36,030 | 36.7506 |
| 5 mol | 90.075 | 90,075 | 91.8765 |
| 10 mol | 180.15 | 180,150 | 183.753 |
Weigh is the amount multiplied by 18.015 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Methane | CH4 | 16.043 | 62.332 |
| Ammonia | NH3 | 17.031 | 58.716 |
| Water (this page) | H2O | 18.015 | 55.509 |
| Carbon monoxide | CO | 28.01 | 35.702 |
| Nitrogen | N2 | 28.014 | 35.696 |
| Oxygen | O2 | 31.998 | 31.252 |
| Hydrogen peroxide | H2O2 | 34.014 | 29.4 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.