The molar mass of hydrogen peroxide (H2O2) is 34.014 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
or
.
| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 5.93% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 94.07% |
| Total | 4 | 34.014 | 100.00% |
2.016 + 31.998 = 34.014 g/mol
Grams and moles
25 g of hydrogen peroxide is 0.7350 mol, because 25 ÷ 34.014 = 0.7350.
These elements have no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: hydrogen is 1.0080, and really lies between 1.00784 and 1.00811; oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Hydrogen peroxide is 34.014 g/mol, exactly 15.999 more than water. The two compounds differ by a single oxygen atom, and almost everything interesting about peroxide follows from that one extra atom being held in an unstable oxygen-to-oxygen bond.
Oxygen accounts for 94.07% of the mass here, against 88.81% in water. Peroxide decomposes to water and oxygen gas, so a sealed bottle slowly builds pressure, and warmth or a trace of transition metal accelerates it dramatically.
Retail concentrations are quoted by volume rather than mass, which confuses the arithmetic. The familiar brown-bottle antiseptic is 3% by mass, about 0.88 moles per litre. Hairdressing peroxide at “20 volume” means one litre releases twenty litres of oxygen gas on full decomposition, which works out to roughly 6% by mass.
Because the molecule is so small and so oxygen-rich, it is one of the few oxidising agents whose only by-product is water.