mL to Moles Calculator

A milliliter is a volume, a mole is a count of particles

A milliliter tells you how much space a liquid occupies, and a mole tells you how many particles are in it, so the two answers are never the same kind of number and the gap between them is where all the chemistry lives. One mole of anything contains 6.02214076 x 10^23 particles, a figure fixed by the 2019 redefinition of the SI mole and published by the NIST mole redefinition page. A 250 milliliter beaker of water and a 250 milliliter beaker of hexane hold the same volume and wildly different numbers of molecules, because the molecules themselves are different sizes and the liquids pack differently.

That is why converting milliliters to moles takes two pieces of information, not one. You need the density of the liquid to turn a volume into a mass, and you need the molar mass to turn a mass into a count of moles. Without the density step, a milliliter is as mute as a pound without a balance reading, and without the molar mass step, a gram of sugar and a gram of salt look identical when they are not.

Both numbers are published in reference tables that any working lab trusts. The NIST Chemistry WebBook entry for water lists its molecular weight as 18.0153, and the same database lists ethanol at 46.0684, acetic acid at 60.0520 and glycerin at 92.0938. Densities come from the chemical record for each substance, where PubChem lists ethanol at 0.7893 grams per milliliter at 20 degrees Celsius. Everything on this page traces back to those two kinds of published figure.

The formula: milliliters times density, divided by molar mass

The whole conversion is one expression:

• Moles = (mL x density) / molar mass
• Mass in grams = mL x density in grams per milliliter
• Moles = mass in grams / molar mass in grams per mole

The units check themselves out. Milliliters times grams per milliliter gives grams, and grams divided by grams per mole gives moles. A 250 milliliter sample of water weighs 250 x 0.9982 = 249.550 grams, and 249.550 / 18.0153 = 13.8521 moles, which is 13,852.11 millimoles and 8.342 x 10^24 molecules.

It helps to remember the reverse direction too, because one mole of a liquid has a volume you can picture. Divide molar mass by density and you get the milliliters that hold one mole: 18.0153 / 0.9982 = 18.05 mL for water, 46.0684 / 0.7893 = 58.37 mL for ethanol, and 92.0938 / 1.2613 = 73.01 mL for glycerin. A 100 milliliter flask of water therefore holds a bit more than five and a half moles, and the same flask of hexane holds only 0.7647 moles, because hexane molecules are heavy and hexane is not dense.

One caution before you start converting. This formula assumes a pure liquid. A solution is a mixture, and its density is not the density of the solvent, so the pure liquid path gives you a wrong answer for a solution. For a solution, moles come from the concentration instead, which is the second half of this page.

Densities and molar masses for eight common liquids

The table below carries the pair of numbers you need for each liquid, and the mole count in a standard 100 milliliter sample. Densities are for 20 degrees Celsius; molar masses come from the NIST Chemistry WebBook entries for each compound.

Two rows in that table carry the lesson. Water is the lightest molecule on the list and the densest liquid in it, so 100 milliliters of water holds 5.5408 moles while 100 milliliters of hexane holds 0.7647. Hexane molecules are large and light together, so a given volume of hexane contains fewer of them, and 100 milliliters of hexane weighs 65.900 grams against 99.820 grams for the same volume of water.

Glycerin shows the other end of the density range at 1.2613 grams per milliliter, so a 15 milliliter dose of it weighs 18.920 grams against 14.973 grams for the same volume of water. It also carries the second heaviest molar mass on the list, and the two effects partly cancel: 15 milliliters of glycerin holds 0.2054 moles, while the same volume of water holds 0.8311 moles, about four times as many particles for less than 80 percent of the mass.

Worked example: 250 milliliters of water, then 50 of ethanol

Start with the case everybody has in front of them, a 250 milliliter beaker of water. The density of water at 20 degrees Celsius is 0.9982 grams per milliliter, and the National Bureau of Standards circular 19 density tables put the same figure at 0.99823 grams per milliliter, so 250 milliliters weighs 250 x 0.9982 = 249.550 grams. Moles are 249.550 / 18.0153 = 13.8521, which is 13,852.11 millimoles, or 13.8521 x 6.02214076 x 10^23 = 8.342 x 10^24 molecules.

Now the same volume of a different liquid, 50 milliliters of ethanol. At 0.7893 grams per milliliter that weighs 50 x 0.7893 = 39.465 grams, and 39.465 / 46.0684 = 0.8567 moles, which is 856.66 millimoles and 5.159 x 10^23 molecules. Half a cup of ethanol therefore carries about 1/16 the molecules of a cup of water, even though the two volumes were identical on the bench.

Small volumes behave the same way, which matters for drops and pipettes. Five milliliters of acetone weighs 5 x 0.7845 = 3.923 grams and holds 3.923 / 58.0791 = 0.0675 moles, or 67.54 millimoles. A single milliliter of water is 0.9982 grams, which is 0.0554 moles, so a milliliter pipette delivers about 55.41 millimoles of water or 1/10 that of a heavier solvent.

Solutions take a different road: moles from molarity

When the liquid is a solution, the solute is what you count and the concentration is what you use. Molarity is moles of solute per liter of solution, so the formula inverts into a multiplication:

• Moles = molarity (mol/L) x volume (L)
• Grams of solute = moles x molar mass of the solute

The same multiplication covers the concentrations that show up on reagent bottles.

A 750 milliliter flask of 0.20 molar acetic acid holds 0.75 x 0.20 = 0.150 moles of acetic acid, which at 60.0520 grams per mole is 9.008 grams of pure acetic acid dissolved in the flask. A 500 milliliter portion of 0.150 molar sodium chloride holds 0.50 x 0.150 = 0.0750 moles, which is 0.0750 x 58.44 = 4.383 grams of salt. A 1.00 molar solution is easy to sanity check: 25 milliliters of it holds 0.0250 moles, and for salt that is 1.461 grams, about a quarter teaspoon.

Notice that the molarity path never uses the density at all, and it should not. The density of a 0.20 molar acetic acid solution is close to the density of water, not the 1.0490 grams per milliliter of pure acetic acid, so running the pure liquid formula over a solution gives a number for the whole mixture rather than the solute. The molarity calculator works through that path in detail, and the molar mass calculator builds the molar mass from a formula you type.

For solid reagents the density step is unnecessary, since you weigh them directly. A gram of sodium chloride divided by 58.44 grams per mole is 0.0171 moles, and 0.0171 moles of salt in 250 milliliters of water gives a 0.0684 molar solution.

Millimoles, micromoles and the small end of the scale

Chemistry stockrooms and biology protocols both work in millimoles and micromoles, and the same division produces them. One millimole is a thousandth of a mole, so multiply the mole count by 1,000. One micromole is a millionth, so multiply by 1,000,000.

Ten milliliters of glycerin is a common dilution step, and it works out to 10 x 1.2613 = 12.613 grams and 12.613 / 92.0938 = 0.1370 moles, or 137.0 millimoles. Spread that across a liter of water and you have a 0.137 molar solution of glycerin.

For a sense of scale, one mole of water occupies 18.05 milliliters, so a single drop of 0.05 milliliters carries about 0.0028 moles of water, which is 2.8 millimoles and roughly 1.7 x 10^21 molecules. The numbers get large quickly because the mole is a counting unit, and that is exactly why the mole is the unit that keeps stoichiometry in small, readable figures.

Where the shortcut breaks: temperature, mixtures and precision

Three things quietly change the answer, and it is worth naming them before you rely on a number from this page.

Temperature changes density. The circular 19 tables show water at 0.99225 grams per milliliter at 40 degrees Celsius, against 0.99823 at 20 degrees. A 250 milliliter sample at 40 degrees therefore weighs 248.06 grams and holds 13.7695 moles, about 0.08 moles less than the same volume at room temperature. Warm a beaker and the mole count of the liquid in it falls.

Mixtures are not pure liquids. Adding salt to water raises the volume a little and the density a lot, and vodka at 40 percent alcohol by volume is not the density of ethanol because the water is still in there. The published ethanol and water density tables in circular 19 exist precisely for this reason, giving 0.99823 grams per milliliter for pure water and lower figures for mixtures at 20 degrees.

Precision follows the input. Densities are usually given to four significant figures, so a mole count derived from them is good to about four significant figures and no further. Reporting 13.8521 moles from a 0.9982 density is honest, while reporting 13.852109 moles pretends to know more than the density table does. Round to the same number of figures as your densest input, and round the final mole count to match.

For the neighbouring jobs, the grams to moles calculator starts from a mass on the scale rather than a volume in a beaker, the density calculator handles the mass, volume and density triangle on its own, and the atoms to moles calculator goes one step further down to individual particles.