The molar mass of methanol, CH4O, is 32.042 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 32.042 g, and a gram of it is 1.879e+22 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Methanol is 12.011 (C) + 4 × 1.008 (H) + 15.999 (O), which is 32.042 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 6 atoms in a formula unit is oxygen, and they account for 15.999 g of the 32.042 g, or 49.93% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of methanol is 32.042 mg and one micromole is 32.042 µg, so a balance reading to a milligram places you within 31.21 µmol. A single formula unit weighs 5.3207e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Wood alcohol: a solvent, a fuel and the feedstock for biodiesel. Toxic by ingestion, absorption and inhalation, unlike the ethanol it resembles. At 32.042 g/mol it is the lightest of the 21 organic compounds covered here, and that ranking matters more than it looks: ethanol has a molar mass of 46.069 g/mol, so a gram of it contains 30.4% fewer formula units than a gram of methanol. Weigh by mass and you are not weighing equal amounts of substance.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 0.64084 g of a nominal 32.042 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
m- The mass of methanol in grams
32.042- The molar mass of methanol in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of methanol in grams.
- It is divided by the molar mass, 32.042 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of methanol as a molecule count
0.5 g divided by 32.042 g/mol is 0.0156045 mol, and multiplying by the Avogadro constant gives 9.397e+21 formula units of methanol. The same mass is 15.605 mmol, which is the figure a reaction is actually planned in.
5 g of methanol as a molecule count
5 g divided by 32.042 g/mol is 0.156045 mol, and multiplying by the Avogadro constant gives 9.397e+22 formula units of methanol. The same mass is 156.05 mmol, which is the figure a reaction is actually planned in.
50 g of methanol as a molecule count
50 g divided by 32.042 g/mol is 1.56045 mol, and multiplying by the Avogadro constant gives 9.397e+23 formula units of methanol. The same mass is 1560.5 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of methanol (CH4O)?
It is 32.042 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 12.011 (C) + 4 × 1.008 (H) + 15.999 (O). One mole of methanol therefore weighs 32.042 g, and a gram of it is 31.209 mmol.
What percentage of methanol is oxygen?
49.93% by mass. Each formula unit contains 1 oxygen atom contributing 15.999 g of the 32.042 g total, so 100 g of methanol contains 49.93 g of oxygen and a kilogram contains 499.31 g of it.
How many molecules are in one gram of methanol?
About 1.879e+22. One gram is 0.031209 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 32.042.
How much does a single molecule of methanol weigh?
5.3207e-23 g, which is 32.042 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for methanol it is 32.042. Molar mass carries grams per mole: 32.042 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why methanol comes out at 32.042 g/mol rather than 32.
Molar mass, composition and mole equivalents of methanol
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 1 | 15.999 | 15.999 | 49.93% |
| Carbon (C) | 1 | 12.011 | 12.011 | 37.49% |
| Hydrogen (H) | 4 | 1.008 | 4.032 | 12.58% |
| Total — one mole of methanol | 32.042 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 32.042 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000032 | 0.032042 | 6.022e+17 |
| 10 µmol | 0.00032042 | 0.32042 | 6.022e+18 |
| 100 µmol | 0.0032042 | 3.2042 | 6.022e+19 |
| 1 mmol | 0.032042 | 32.042 | 6.022e+20 |
| 10 mmol | 0.32042 | 320.42 | 6.022e+21 |
| 50 mmol | 1.6021 | 1602.1 | 3.011e+22 |
| 100 mmol | 3.2042 | 3204.2 | 6.022e+22 |
| 250 mmol | 8.0105 | 8010.5 | 1.506e+23 |
| 500 mmol | 16.021 | 16,021 | 3.011e+23 |
| 750 mmol | 24.0315 | 24,031.5 | 4.517e+23 |
| 1 mol | 32.042 | 32,042 | 6.022e+23 |
| 2 mol | 64.084 | 64,084 | 1.204e+24 |
| 5 mol | 160.21 | 160,210 | 3.011e+24 |
| 10 mol | 320.42 | 320,420 | 6.022e+24 |
Mass is the amount multiplied by 32.042 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Methanol (this page) | CH4O | 32.042 | 31.209 |
| Ethanol | C2H6O | 46.069 | 21.707 |
| Acetone | C3H6O | 58.08 | 17.218 |
| Urea | CH4N2O | 60.056 | 16.651 |
| Isopropyl alcohol | C3H8O | 60.096 | 16.64 |
| Ethylene glycol | C2H6O2 | 62.068 | 16.111 |
| Glycine | C2H5NO2 | 75.067 | 13.321 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.