Chemical formula of aspirin (C₉H₈O₄)
Aspirin is acetylsalicylic acid, formula C₉H₈O₄, molar mass 180.16 g/mol. By coincidence that is the same molar mass as glucose, from a completely different set of atoms.
Updated on October 7, 2026
Formula, molar mass and composition
C₉H₈O₄
180.159 g/mol± 0.0185
- Formula
- C₉H₈O₄
- IUPAC
- acetylsalicylic acid
- Name
- aspirin
- Molar mass
- 180.159 g/mol
Composition by mass
| Symbol | Name | Atoms | Standard atomic weight | Share of mass |
|---|---|---|---|---|
| C | Carbon | 9 | 12.011 | 60.00 % |
| H | Hydrogen | 8 | 1.008 | 4.48 % |
| O | Oxygen | 4 | 15.999 | 35.52 % |
IUPAC publishes a range for this element because its isotopic composition varies in nature. The value shown is the conventional atomic weight, and the uncertainty is half the width of that range, not a measurement error.
From a tablet to moles
A standard 500 mg tablet contains 0.5 / 180.16 = 2.78 millimoles of acetylsalicylic acid, about 1.67 × 10²¹ molecules. Low-dose "baby" aspirin, 81 or 100 mg, works out at 0.45 and 0.56 millimoles.
Acetylsalicylic acid is made by acetylating salicylic acid, C₇H₆O₃, which occurs in willow bark and was the traditional remedy the drug came from. This page is about the chemistry only: nothing here is medical advice, and aspirin has real contraindications.
Source: Cálculo próprio a partir dos dados citados nesta página
How to read this page
How to read the numbers here
Every value on "Chemical formula of aspirin (C₉H₈O₄)" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "Chemical formula of aspirin (C₉H₈O₄)" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
C9H8O4’s molar mass shifts slightly with where the sample comes from, because one of its elements has an atomic weight published as a range, not a single value.
More about “Chemical formula of aspirin (C₉H₈O₄)”
C9H8O4 compared with water
At 180.159 g/mol, one molecule (or formula unit) of C9H8O4 weighs 10.0 times as much as one water molecule. A single gram of C9H8O4 holds roughly 3.3427 × 10²¹ units — a number that only makes sense because the mole exists.
Worth repeating, because "Chemical formula of aspirin (C₉H₈O₄)" depends on it: one element in C9H8O4 has an atomic weight published as a range by the IUPAC, so the third decimal place of the molar mass shifts a little with where the sample came from.
Words on this page
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Isotope
Atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass. Carbon-12 and carbon-14 are isotopes of carbon; only the second one is radioactive.
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number