The molar mass of sulfur dioxide, SO2, is 64.058 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 64.058 g, and a gram of it is 9.401e+21 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. Sulfur dioxide is 32.06 (S) + 2 × 15.999 (O), which is 64.058 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Sulfur makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is sulfur, and they account for 32.06 g of the 64.058 g, or 50.05% 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 sulfur dioxide is 64.058 mg and one micromole is 64.058 µg, so a balance reading to a milligram places you within 15.61 µmol. A single formula unit weighs 1.0637e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The wine preservative and the gas of coal smoke, measured in parts per million in air and in mg per litre in wine. At 64.058 g/mol it is the heaviest of the 11 gases and small molecules covered here, and that ranking matters more than it looks: butane has a molar mass of 58.124 g/mol, so a gram of it contains 10.2% more formula units than a gram of sulfur dioxide. 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 64.058 g of sulfur dioxide — 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
m- The mass of sulfur dioxide in grams
64.058- The molar mass of sulfur dioxide 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 sulfur dioxide in grams.
- It is divided by the molar mass, 64.058 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 sulfur dioxide as a molecule count
0.5 g divided by 64.058 g/mol is 0.00780543 mol, and multiplying by the Avogadro constant gives 4.701e+21 formula units of sulfur dioxide. The same mass is 7.8054 mmol, which is the figure a reaction is actually planned in.
5 g of sulfur dioxide as a molecule count
5 g divided by 64.058 g/mol is 0.0780543 mol, and multiplying by the Avogadro constant gives 4.701e+22 formula units of sulfur dioxide. The same mass is 78.054 mmol, which is the figure a reaction is actually planned in.
50 g of sulfur dioxide as a molecule count
50 g divided by 64.058 g/mol is 0.780543 mol, and multiplying by the Avogadro constant gives 4.701e+23 formula units of sulfur dioxide. The same mass is 780.54 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of sulfur dioxide (SO2)?
It is 64.058 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 32.06 (S) + 2 × 15.999 (O). One mole of sulfur dioxide therefore weighs 64.058 g, and a gram of it is 15.611 mmol.
What percentage of sulfur dioxide is sulfur?
50.05% by mass. Each formula unit contains 1 sulfur atom contributing 32.06 g of the 64.058 g total, so 100 g of sulfur dioxide contains 50.05 g of sulfur and a kilogram contains 500.48 g of it.
How many molecules are in one gram of sulfur dioxide?
About 9.401e+21. One gram is 0.0156109 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 64.058.
How much does a single molecule of sulfur dioxide weigh?
1.0637e-22 g, which is 64.058 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 sulfur dioxide it is 64.058. Molar mass carries grams per mole: 64.058 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. Sulfur is quoted as 32.06 rather than a whole number for that reason, and the same applies to the other elements here, which is why sulfur dioxide comes out at 64.058 g/mol rather than 64.
Molar mass, composition and mole equivalents of sulfur dioxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Sulfur (S) | 1 | 32.06 | 32.06 | 50.05% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 49.95% |
| Total — one mole of sulfur dioxide | 64.058 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 64.058 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000064 | 0.064058 | 6.022e+17 |
| 10 µmol | 0.00064058 | 0.64058 | 6.022e+18 |
| 100 µmol | 0.0064058 | 6.4058 | 6.022e+19 |
| 1 mmol | 0.064058 | 64.058 | 6.022e+20 |
| 10 mmol | 0.64058 | 640.58 | 6.022e+21 |
| 50 mmol | 3.2029 | 3202.9 | 3.011e+22 |
| 100 mmol | 6.4058 | 6405.8 | 6.022e+22 |
| 250 mmol | 16.0145 | 16,014.5 | 1.506e+23 |
| 500 mmol | 32.029 | 32,029 | 3.011e+23 |
| 750 mmol | 48.0435 | 48,043.5 | 4.517e+23 |
| 1 mol | 64.058 | 64,058 | 6.022e+23 |
| 2 mol | 128.116 | 128,116 | 1.204e+24 |
| 5 mol | 320.29 | 320,290 | 3.011e+24 |
| 10 mol | 640.58 | 640,580 | 6.022e+24 |
Mass is the amount multiplied by 64.058 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 |
|---|---|---|---|
| Nitrogen | N2 | 28.014 | 35.696 |
| Oxygen | O2 | 31.998 | 31.252 |
| Hydrogen peroxide | H2O2 | 34.014 | 29.4 |
| Carbon dioxide | CO2 | 44.009 | 22.723 |
| Propane | C3H8 | 44.097 | 22.677 |
| Butane | C4H10 | 58.124 | 17.205 |
| Sulfur dioxide (this page) | SO2 | 64.058 | 15.611 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.