The molar mass of aspirin (C9H8O4) is 180.159 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 |
|---|---|---|---|---|
| Carbon (C) | 9 | 12.011 | 108.099 | 60.00% |
| Hydrogen (H) | 8 | 1.0080 | 8.064 | 4.48% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 35.52% |
| Total | 21 | 180.159 | 100.00% |
108.099 + 8.064 + 63.996 = 180.159 g/mol
Grams and moles
25 g of aspirin is 0.1388 mol, because 25 ÷ 180.159 = 0.1388.
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: carbon is 12.011, and really lies between 12.0096 and 12.0116; 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.
Aspirin is 180.159 g/mol, which lands remarkably close to glucose at 180.156. Three thousandths of a gram per mole separate a painkiller from a sugar, and no balance in a teaching laboratory will tell them apart by weight.
The two are not isomers. Glucose is C6H12O6 and aspirin is C9H8O4, different atoms in different numbers that happen to sum to almost the same total. Their percent compositions are nothing alike: aspirin is 60.00% carbon against glucose’s 40.00%. It is a useful reminder that a molar mass is a sum and sums collide.
The standard tablet is 300 to 325 mg, which is about 1.7 millimoles. A low-dose cardiac tablet at 75 mg is around 0.42 millimoles.
Aspirin is made by acetylating salicylic acid, and the synthesis is a common first-year practical precisely because the yield can be checked by weighing and converting back to moles.