The molar mass of hydrogen peroxide, H2O2, is 34.014 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 34.014 g, and a gram of it is 1.77e+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. Hydrogen peroxide is 2 × 1.008 (H) + 2 × 15.999 (O), which is 34.014 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: 2 of the 4 atoms in a formula unit are oxygen, and they account for 31.998 g of the 34.014 g, or 94.07% 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 hydrogen peroxide is 34.014 mg and one micromole is 34.014 µg, so a balance reading to a milligram places you within 29.4 µmol. A single formula unit weighs 5.6482e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Sold at 3% for first aid, 6% for hair and 35% for industry. It decomposes to water and oxygen, faster when warm or contaminated. At 34.014 g/mol it is heavier than 6 of the 11 gases and small molecules covered here, and that ranking matters more than it looks: oxygen has a molar mass of 31.998 g/mol, so a gram of it contains 6.3% more formula units than a gram of hydrogen peroxide. 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 34.014 g of hydrogen peroxide — 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 hydrogen peroxide in grams
34.014- The molar mass of hydrogen peroxide 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 hydrogen peroxide in grams.
- It is divided by the molar mass, 34.014 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 hydrogen peroxide as a molecule count
0.5 g divided by 34.014 g/mol is 0.0146998 mol, and multiplying by the Avogadro constant gives 8.852e+21 formula units of hydrogen peroxide. The same mass is 14.7 mmol, which is the figure a reaction is actually planned in.
5 g of hydrogen peroxide as a molecule count
5 g divided by 34.014 g/mol is 0.146998 mol, and multiplying by the Avogadro constant gives 8.852e+22 formula units of hydrogen peroxide. The same mass is 147 mmol, which is the figure a reaction is actually planned in.
50 g of hydrogen peroxide as a molecule count
50 g divided by 34.014 g/mol is 1.46998 mol, and multiplying by the Avogadro constant gives 8.852e+23 formula units of hydrogen peroxide. The same mass is 1470 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of hydrogen peroxide (H2O2)?
It is 34.014 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 1.008 (H) + 2 × 15.999 (O). One mole of hydrogen peroxide therefore weighs 34.014 g, and a gram of it is 29.4 mmol.
What percentage of hydrogen peroxide is oxygen?
94.07% by mass. Each formula unit contains 2 oxygen atoms contributing 31.998 g of the 34.014 g total, so 100 g of hydrogen peroxide contains 94.07 g of oxygen and a kilogram contains 940.73 g of it.
How many molecules are in one gram of hydrogen peroxide?
About 1.77e+22. One gram is 0.0293997 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 34.014.
How much does a single molecule of hydrogen peroxide weigh?
5.6482e-23 g, which is 34.014 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 hydrogen peroxide it is 34.014. Molar mass carries grams per mole: 34.014 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 hydrogen peroxide comes out at 34.014 g/mol rather than 34.
Molar mass, composition and mole equivalents of hydrogen peroxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 2 | 15.999 | 31.998 | 94.07% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 5.927% |
| Total — one mole of hydrogen peroxide | 34.014 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 34.014 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000034 | 0.034014 | 6.022e+17 |
| 10 µmol | 0.00034014 | 0.34014 | 6.022e+18 |
| 100 µmol | 0.0034014 | 3.4014 | 6.022e+19 |
| 1 mmol | 0.034014 | 34.014 | 6.022e+20 |
| 10 mmol | 0.34014 | 340.14 | 6.022e+21 |
| 50 mmol | 1.7007 | 1700.7 | 3.011e+22 |
| 100 mmol | 3.4014 | 3401.4 | 6.022e+22 |
| 250 mmol | 8.5035 | 8503.5 | 1.506e+23 |
| 500 mmol | 17.007 | 17,007 | 3.011e+23 |
| 750 mmol | 25.5105 | 25,510.5 | 4.517e+23 |
| 1 mol | 34.014 | 34,014 | 6.022e+23 |
| 2 mol | 68.028 | 68,028 | 1.204e+24 |
| 5 mol | 170.07 | 170,070 | 3.011e+24 |
| 10 mol | 340.14 | 340,140 | 6.022e+24 |
Mass is the amount multiplied by 34.014 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 |
|---|---|---|---|
| Carbon monoxide | CO | 28.01 | 35.702 |
| Nitrogen | N2 | 28.014 | 35.696 |
| Oxygen | O2 | 31.998 | 31.252 |
| Hydrogen peroxide (this page) | H2O2 | 34.014 | 29.4 |
| Carbon dioxide | CO2 | 44.009 | 22.723 |
| Propane | C3H8 | 44.097 | 22.677 |
| Butane | C4H10 | 58.124 | 17.205 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.