Molar MassMolar mass of hydrogen peroxide (H2O2)
The molar mass of hydrogen peroxide, H2O2, is 34.01 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 34.01 grams.
About hydrogen peroxide
Hydrogen peroxide is sold as a dilute solution for bleaching and as an antiseptic. It breaks down into water and oxygen. It has one more oxygen atom than water, which nearly doubles the molar mass.
The working
The formula H2O2 has 2 hydrogen atoms and 2 oxygen atoms, 4 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 5.93% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 94.07% |
2 × 1.0080 + 2 × 15.999 = 34.014 g/mol
With the rounded atomic masses used in many school books (H = 1, O = 16) the answer is 34 g/mol. The difference from 34.01 is too small to matter in most school sums.
Percentage composition
By mass, H2O2 is 5.93% hydrogen and 94.07% oxygen. The largest share comes from oxygen. In a 100 g sample there are 5.9 g of hydrogen and 94.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 50 g of H2O2? Divide the mass by the molar mass: 50 ÷ 34.01 = 1.47 mol.
Moles to grams. What is the mass of 0.4 mol of H2O2? Multiply: 0.4 × 34.01 = 13.61 g.
Counting particles. 50 g contains 1.47 mol × 6.022 × 1023 = 8.852 × 1023 molecules, and 3.541 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 3.401 g |
| 0.5 mol | 17.01 g |
| 1 mol | 34.01 g |
| 2 mol | 68.03 g |
| 5 mol | 170.1 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0294 mol |
| 10 g | 0.294 mol |
| 100 g | 2.94 mol |
| 1 kg | 29.4 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of H2O2?
The molar mass of hydrogen peroxide (H2O2) is 34.01 g/mol, or 34.014 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of H2O2?
100 ÷ 34.01 = 2.94 mol.
What is the mass of one molecule of H2O2?
Divide the molar mass by the Avogadro constant: 34.01 ÷ 6.022 × 1023 = 5.648 × 10-23 g.
What is the percentage of oxygen in H2O2?
Oxygen makes up 94.07% of the mass of H2O2: 2 × 15.999 ÷ 34.014 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.